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Elementary _ 






er Photographic 
1 Chemistry 


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Eastman Kodak Company 
| Rochester, N. Y. | 


RESEARCH LIBRARY 
THE GETTY RESEARCH INSTITUTE 


JOHN MOORE ANDREAS COLOR CHEMISTRY LIBRARY FOUNDATION 





Elementary 
Photographic 
Chemistry 


Eastman Kodak Company 
Rochester, N. Y. 
1924 


THE 


INST 





CONTENTS 


Chapter. 
I. An Outline of Elementary Chemistry- - - 
II. The Chemistry of Photographic Materials-  - 
III. The Chemistry of Development - - - - 
IV. The Chemistry of Fixation - - - - - 
V. The Chemistry of Toning - - ee 
VI. The Chemistry of Intensification and enue 
VII. The Chemistry of Washing - - - - - 
Mite eormilas «is so ee eee 
IX. Preparing Solutions TN RAY RU EE TO tial > 
X. Simple Chemical Tests - - - - - - 


TS al es a 


INTRODUCTION 


Photography is so essentially a chemical process that every 
photographer should have an interest in the chemicals which 
he uses and in the reactions which they undergo. 


This book is written in response to a demand for a simple 
account of photographic chemistry, for the practical photog- 
rapher. 


No attempt has been made to give the chemical theory 
in full, for which textbooks on chemistry should be consulted. 
In Chapter I, a statement is given only of the chemistry 
which is necessary to an understanding of the remainder of 
the book. In the same way reference should be made to photo- 
graphic textbooks for general photographic practice, as this 
book treats only of photographic chemistry and not practical 
photography. 

In order to give the information about photographic chem- 
icals which is necessary for their intelligent use, the properties 
of each of the more important chemicals are given in a separate 
paragraph which is inserted in the section dealing with its 
use but is printed in a smaller distinct type face to facilitate 
reference; an index to the chemicals dealt with is given at 
the end of the book. 


No apology is needed for the insistence placed on the need 
for pure chemicals and on the advantage to be gained by using 
the Eastman Tested Chemicals, which are specially purified 
and tested for photographic use. 

EASTMAN KODAK COMPANY, 
RocuHEsTER, N. Y. 


April, 1924 


CHAPTER I. 


An Outline of Elementary Chemistry 


All substances are made by the combination in various pro- 
portions of a limited number of elements, of which about 
eighty exist. These elements combine in definite proportions 
to form bodies of fixed composition, which are termed com- 
pounds. ‘Thus, one volume of the gaseous element hydrogen 
combines with one volume of the gaseous element chlorine to 
form two volumes of the compound hydrochloric acid gas. 
This combination can be represented by what is called a 
chemical equation. Thus, if we write H for hydrogen, Cl for 
chlorine and H Cl for hydrochloric acid, we can represent the 
above combination by the equation 


H + Cl HCl 
Hydrogen Chlorine Hydrochloric Acid Gas 


It will be seen that an equation such as that given above is 
really a shorthand method of stating what happens, the ele- 
ments which take part in the combination being designated by 
letters. These letters which stand for the elements are called 
the “symbols” of the elements. 


The elements which are of the greatest importance in 
photography and their symbols are as follows: 


Gases 
Name. Symbol. Remarks. 

Hydrogen H_ The lightest gas known. 

Nitrogen N_ Forms 80% of the air. 
(Approx.) 

Oxygen O Forms 20% of the air. 
(Approx.) 

Chlorine Cl Greenish - yellow poisonous 
gas. 

Bromine Br Poisonous brownish-red gas 


at high temperatures, liquid 
at ordinary temperatures. 


6 EASTMAN KODAK COMPANY 
Non-metallic Solids 
Name. Symbol. Remarks. 

Carbon C Occurs in three forms: dia- 
mond, graphite, and char- 
coal or amorphous carbon. 

Sulphur S  Yellowish-white, brittle solid. 

Iodine I Violet plate-like crystals, sim- 
ilar in chemical properties 
to chlorine and bromine. 

Metallic Solids 

Sodium Na_ Very light, attacked by mois- 
ture, kept under light oil. 

Potassium K_ Very light, attacked by mois- 
ture, kept under light oil. 

Calcium Ca _ Silvery white metal, attacked 
by moisture. 

Aluminum Al Very light, white metal. 

Iron Fe In the pure state it is called 

) wrought-iron; when con- 
taining a small amount of 
carbon it forms cast-iron 
and steel. 

Copper Cu Reddish, tough metal. 

Silver Ag. White metal. 

Platinum Pt Valuable white metal, very 
heavy. 

Gold Au Reddish yellow metal, very 
heavy. 

Mercury Hg White metallic liquid, very 
heavy 


* 


These elements fall into two groups; those which are 
metals and those which are not metals. Apart from the ap- 


pearance of the elements, the classification of an element in one 
of these two groups depends upon its relation to oxygen. Many 
of the elements when heated in the presence of oxygen will 
combine with it and will form what are called oxides. Thus, 
carbon will burn in oxygen and will form a gaseous compound 
of carbon with oxygen called carbon dioxide. Iron will burn 
in oxygen and forms a solid iron oxide. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 7 


Oxides of Elements 


Name. Symbol. Remarks. 
Hydrogen oxide (water) HzO Can be made by burning hy- 
drogen in air or oxygen. 


Acid Oxides 


Carbon dioxide CO. A heavy gas, is produced by 
burning carbon; e. g., char- 
coal. 

Nitric oxide NO Colorless gas, turns reddish- 

brown in contact with 
oxygen. 

Sulphur dioxide SO. Colorless gas. Produced by 


burning sulphur. 
Basic Oxides 


Aluminum oxide AlzO; White powder which forms 
when aluminum is burned 
in the air. 

Calcium oxide CaO Quicklime, obtained by heat- 
ing chalk. 

Iron oxide Fe20; Red powder formed when 
iron rusts. 

Mercuric oxide HgO Red powder formed by slow 
heating of mercury in the 
air. 


Many oxides are soluble in water, forming two classes of 
compounds, which are known respectively as acids and bases, 
the acid oxides being produced from the non-metallic elements 
and the basic oxides from the metallic elements. Thus, car- 
bon, nitrogen and sulphur all form acid oxides which dissolve 
in water to form acids, while sodium, potassium and calcium 
form typical basic oxides which dissolve in water to form 
bases. 


Bases are either alkaline or earthy, the alkaline bases being 
soluble, the earthy bases insoluble. The ordinary way of 
distinguishing between an acid and a base is to test the solution 
with a trace of certain dyes which change color according to 
whether the solution is acid or alkaline. Thus, if a piece of 
paper soaked in a solution of litmus, generally known as 
litmus paper, is put into a solution, it will turn red if the solu- 
tion is acid, and blue if the solution is alkaline. Thus, sodium 
forms an oxide which dissolves in water and makes a solution 


8 EASTMAN KODAK COMPANY. 


of basic caustic soda, the caustic soda having the formula 
NaOH, and being composed of sodium, oxygen and hydrogen. 
On the other hand, sulphur combines with oxygen and the 
oxide dissolves in water to form sulphurous acid, this having 
the formula H:SO3 and being formed by the combination of 
water, H.O, with sulphur dioxide, SOz. Thus: 
SO2 + HO = H2SOz3 
Sulphur Dioxide Water Sulphurous Acid 

All acids contain hydrogen and this hydrogen can be re- 
placed by a metal, forming a compound which is termed a 
“‘salt.’? Thus, if we have sulphuric acid and we dissolve a piece 
of iron in it, the iron will replace the hydrogen of the acid, 
which will be given off as bubbles of gas and a solution of the 
salt, iron sulphate, will be formed: 

H2S04 + Fe = Fe SO.4 + He 
Sulphuric Acid Iron _ Iron Sulphate Hydrogen Gas 

Salts are also formed by the direct union of an acid and a 
base. Thus, if we have caustic soda, NaOH, and sulphurous 
acid, H:SO3, they combine to form sodium sulphite, eliminat- 
ing water. Thus: 

2NaOH oo H2SO3 = NasSO3 oa 2H20 


Two parts of Sulphurous Acid Sodium Sulphite Water 
Caustic Soda 


It will be seen that the sodium sulphite is formed by the com- 
bination of the base derived from sodium with the acid derived 
from sulphur. 

Sometimes a non-metallic element forms two different 
oxides, and these in turn will form two different acids. When 
we burn sulphur in oxygen, for instance, each atom of sulphur 
combines with two atoms of oxygen and forms sulphur di- 
oxide: 


S +20 = SO2 


and this dissolves in water to form sulphurous acid. If the 
sulphur dioxide is passed, with more oxygen over heated 
platinum, it is possible to make it combine with another atom 
of oxygen and form the compound sulphur trioxide, SOs, and 
this dissolves in water and forms sulphuric acid: 


SO3 + H20 = H2S0.4 


so that from sulphur we not only get sulphurous acid but a 
second acid—sulphuric acid. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 9 


Just as the hydrogen of sulphurous acid is replaced by 
sodium to form sodium sulphite, so the hydrogen of sulphuric 
acid is replaced by sodium to form sodium sulphate. 


Sulphur Dioxide SO2 Sulphur Trioxide SO3 
Sulphurous Acid H2S8O3 Sulphuric Acid H2S04 
Sodium Sulphite Naz2SOz2 Sodium Sulphate Na2SO4 


Salts are usually neutral to litmus paper, though sometimes 
they are somewhat acid or alkaline. But in addition to the 
neutral salts, an acid in which there are two hydrogen atoms 
can have one of them replaced by a metal instead of both, and 
in this case we get acid salts, which are equivalent in their 
behavior to a mixture of equal parts of the acid and the neutral 
salt. For instance, from sulphurous acid if we replace both 
the hydrogens, we get sodium sulphite—Na2SO;—but if we 
replace only one of the hydrogens, we get the compound 
NaHSOs, which is called sodium acid sulphite, sodium hydro- 
gen sulphite or, more usually, sodium bisulphite. 


Sulphur forms a number of different acids. It forms not 
only acids from its two oxides SO2 and SOs, but it forms com- 
pound acids containing more than one atom of sulphur, and of 
these one is a very great importance to the photographer, 
namely, thiosulphuric acid, which forms a sodium salt, sodium 
thiosulphate, NazS2.03. It will be seen that this compound 
differs from sodium sulphite in having two atoms of sulphur 
instead of one, and it is the compound, generally known as 
“hypo,” which is used for fixing photographic materials. 


Some acids are formed not from oxides but by the direct 
combination of a non-metallic element with hydrogen, and of 
these the most important are the strong acids formed from 
chlorine, bromine and iodine, which three elements, because 
they occur in sea salt, are called halogens, from the Greek 
name for the salt sea. Thus, chlorine combines directly with 
hydrogen to form hydrochloric acid, H Cl, and if the hydrogen 
of this is replaced by metals, we get chlorides, of which the 
best known is sodium chloride, Na Cl, which is common salt. 
Similarly, bromine combines with hydrogen to form hydro- 
bromic acid, with which metals form bromides, and in the same 
way the iodides are formed from iodine. 


10 EASTMAN KODAK COMPANY. 


The Halogens, Their Acids and Salts 


Halogen Element Acid Sodium Salt 

Cl Chlorine H Cl Hydrochloric Acid Na Cl Sodium Chloride 
Br Bromide H Br Hydrobromic Acid Na Br Sodium Bromide 
I Iodine HI Hydriodie Acid NaI Sodium Iodide 


Salts are soluble in water to different extents, the solubil- 
ity depending upon the nature of the salt. Some, such as hypo, 
are extremely soluble, hypo being soluble in less than its own 
volume of water, while others are only slightly soluble or even 
almost completely insoluble, silver chloride, bromide and iodide 
being well known examples of very insoluble materials. A solu- 
tion of a salt may be regarded as containing both the basic and 
the acid components of the salt in a more or less free condition. 
For instance, all copper salts in solution behave in much the 
same way, showing properties in common, due to the presence 
of the copper. In the same way all chlorides or sulphates show 
common properties in solution. 


Now, when we mix two solutions of soluble salts, and the 
base of one can form an insoluble salt with the acid of the 
other, then this rearrangement will take place and the insoluble 
substance will be thrown out of solution as a precipitate. Thus, 
silver nitrate and sodium chloride are both very soluble in 
water, but when the solutions are mixed the silver and the 
sodium change places so that silver chloride and sodium nitrate 
are formed, and the almost insoluble silver chloride is thrown 
out of the solution, leaving only the sodium nitrate behind. 


Ag NOs + Na Cl = Ag Cl + Na NO3 
Silver Nitrate Sodium Chloride Silver Chloride Sodium Nitrate 
Soluble — Soluble Insoluble Soluble 


Precipitated 


This ‘‘double decomposition” is the simplest kind of chemical 
reaction and is the one with which we are most familiar. 


Other types of chemical reaction which are of great im- 
portance in photography are those of oxidation and reduction. 
The simplest example of oxidation is, of course, that in which 
an element combines with oxygen; but when an element forms 
two or more compounds with oxygen, then we are said to per- 
form oxidation when we raise the element from the level of 
oxidation of one of its compounds to another level in which 
it is combined with more oxygen. For example, by the oxida- 
tion of sodium sulphite, NazSO3, which is a compound formed 
from sulphur dioxide, SO2, we get sodium sulphate Na2SOu,, 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 11 


which is derived from sulphur trioxide, SO3. This can be 
done by means of oxygen. If we pass air, which contains 20% 
of oxygen (the rest being chiefly nitrogen), through a sulphite 
solution, or even leave sodium sulphite exposed to the air for 
long periods, it will be oxidized into sulphate— 


NaeSO3 + O = NaeSOu 
Sodium Sulphite Oxygen Sodium Sulphate 

When metallic elements form two oxides with different 
amounts of oxygen, these two oxides will act as bases for two 
series of salts. Thus, iron forms 


Ferrous salts derived from Fe O, and 
Ferric salts derived from Fe20s. 


Thus, we have 
Ferrous Chloride, Fe Cle, green crystals, 
Ferric Chloride, Fe Cl;, red-brown crystals. 


Very often oxidation is accomplished not by the use of oxygen 
itself but by the use of some substance which itself is a higher 
compound of oxygen and which can be reduced to a lower com- 
pound of oxygen or to an element which contains no oxygen 
at all. Thus, for instance, when hydroquinone’ is oxidized, we 
get quinone, which we call the oxidation product of hydro- 
quinone, but if we add sulphite to quinone, the quinone ox- 
idizes the sulphite to sulphate and is itself reduced again to 
hydroquinone. In this case the sulphite acts as a reducing 
agent, reduction being the opposite to oxidation. Thus, a body 
which is easily oxidized will take the oxygen it needs from 
other substances and so acts as a reducing agent. Hydro- 
quinone is oxidized to quinone, which is reduced by sulphite 
to hydroquinone (hydrochinon). Conversely, the sulphite is 
oxidized by the quinone to sulphate. 


Similarly, if we add ferric salts to hydroquinone, they will 
oxidize it to quinone and will themselves be reduced to ferrous 
salts. 


The term reduction is applied especially to the liberation of 
metallic elements from their compounds. Thus, if we heat 
mercuric oxide, the oxygen is driven off by the heat and the 
mercuric oxide is reduced to mercury. Generally, reduction 
cannot be accomplished by heat alone, and it is necessary to 
have some substance present which can be oxidized in order to 


ee ’ 
*Chemists in America and Great Britain spell hydrochinon as hydroquinone. The spelling 
used generally by photographers is hydrochinon. 


12 


EASTMAN KODAK COMPANY. 


reduce a compound. Thus, to reduce iron from its oxide, of 
which iron ore is chiefly composed, we heat it with charcoal or 
carbon, which is oxidized to form carbon dioxide and which 
reduces the iron oxide to metallic iron. 


Chemical compounds consist of five great classes: 





1. ACIDS, which are formed from non-metallic elements and 
: which contain hydrogen replaceable by a metal; 

2. BASES, which are formed from the metallic elements, and 
which, when soluble in water, are called alkalis; 

3. SALTS, which are formed from the combination of an acid 
and a base; 

4. OXIDIZERS, which are substances containing an excess of 
oxygen and which can give up this oxygen to another com- 
pound; 

5. REDUCERS, which are greedy for oxygen and which take 
the oxygen away from any compound containing an avail- 
able supply of it. 

ELEMENTS 
METALS NON-METALS 

form Oxides 
with Oxygen with Oxygen 

“ form Oxides 

ith H 
Oxides with \ Oxides with 
water form form water form 
Acids 
ee 
Salts 





* 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 13 


CHAPTER II. 


‘The Chemistry of Photographic Materials 


The art of photography is founded upon the fact that the 
compounds of silver, and especially its compounds with chlo- 
rine, bromine and iodine, are sensitive to light. 


The earliest photographs were made by coating paper with 
silver chloride and using this to form images by its darkening 
under the action of light, but the sensitiveness of the silver 
chloride was too slight to use it in this way to form images 
in the camera. 


To get results which require less exposure to light, advan- 
tage is taken of the fact that it is not necessary for the light 
to do the whole work of forming the image; it is possible to 
expose the silver salt for only a short time to the light and 
then to continue the production of the image by chemical 
action, the process being termed ‘“‘development.”’ 


Sensitive photographic materials therefore consist of 
paper, film, or glass coated with a layer in which is suspended 
the sensitive silver bromide or silver chloride. This layer is 
called the emulszon. ‘This emulsion consists of a suspension of 
the silver salt in a solution of gelatin. It is made by soaking 
gelatin in water until it is swollen and then dissolving it by 
gently warming and stirring. The necessary bromide or chlo- 
ride, e. g., potassium bromide or sodium chloride, is then added 
to the solution and dissolves in it. Meanwhile; the right 
amount of silver nitrate to react with the amount of salts used 
has been weighed out and is dissolved in water. The silver 






nitrate solution is then added slowly to the solution of gelatin 
and salt and produces in it a precipitate of the silver compound, 


the mixing being done in the dark-room, since the silver com- 
pound produced is sensitive to light. IfsiHere were no gelatin 
in the solution the silver compound would settle down to the 
bottom and an emulsion would not be formed, but the gelatin 
prevents the settling so that as the silver nitrate is added a 
little at a time evenly precipitated silver salt is uniformly 
distributed through the solution. If this emulsion is coated on 
a support, such as paper or film and then cooled, the gelatin 
will set.,to nd when the jelly is dried we get a smooth 
coating of lsion of the sensitive silver compound. 












@ 


14 EASTMAN KODAK COMPANY. 


Photographic materials which are to be developed must 
contain no excess of soluble silver and the emulsion must be 
made so that there is always an excess of bromide or chloride, 
since any excess of soluble silver will produce a heavy fog over 
the whole of the surface as soon as the material is placed in the 
developer. In the case of Solio paper, however, which is not used 
for development but which is printed out, a chloride emulsion 
is made with an excess of silver nitrate. This causes rapid 
darkening in the light, so that prints are made upon Solio paper 
and not developed, the visible image being toned and fixed. 
Solio paper can be developed with certain precautions, such 
as the use of acid developers or after treatment with bromide 
to remove the excess of silver nitrate. 


In the early days of photography prints were usually made 
on printing-out papers, but at the present time most prints 
are made by the use of developing-out chloride and bromide 
papers, which are chemically of the same nature as the negative 
making materials and are coated with emulsions containing no 
free silver nitrate. 


Negative making materials such as plates and films, always 
contain silver bromide with a small addition of silver iodide. 
The different degrees of sensitiveness are obtained by the tem- 
perature and theduration,of heat which the emulsions undergo 
during manufacture, the most sensitive emulsiac ns s being heated 
to higher temperatures and for a longer t the 
emulsions. 


If a slow nai is coatec 

“rial is wn as bromide paper and is used for pri noting 
especial fo making enlarge me Bis. The less sensi 
which commonly used for eo: @ 

light contain silver chloride in t 
















In‘order to obtain silver nitrate the firs vie a is to dissoly 
metallic silver in nitric acid. The silver replaces the hye 
of the acid and fo ilver nitrate, th 


composing a furdesportion of the 
nitrate is crystallizéd out of the solution a 


less, transparent plates 
SILVER NITRATE for photographie has to be extremely 
pure, and since metallic silver usually cont a small quantity of 
other metals, such as copper and lead, it is necess 
these impurities. This is accomplished by recr ion, so that 
the silver nitrate is finally obtained in a perfectl m. 















ind obtain in color- 


O free it from 





* 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 15 


In order to ensure the purity of the silver nitrate which it uses, 
the Eastman Kodak Company prepares its own and is the largest 
maker of silver nitrate in the world, using about one-twenty-fifth of 
all the silver mined in the United States, the mint being the only 
larger consumer. 


Silver nitrate is very soluble in water. It attacks organic material, 
and blackens skin, wood, cloth, and other similar substances on expo- 
sure to light. 


When a solution of silver nitrate is added to a solution of 
a bromide or chloride of another element, a reaction occurs 
and the insoluble silver bromide or chloride is precipitated. 
Thus, if we add silver nitrate to potassium bromide, the re- 
action occurs according to the following equation: 


Ag NOs +e K Br ss Ag Br 4. KNOs 
Silver Nitrate Potassium Bromide Silver Bromide Potassium Nitrate 


The potassium nitrate formed remains in solution, but if the 
solution is at all concentrated, the silver bromide is thrown 
down to the bottom of the vessel as a thick, curdy precipitate. 


The bromides and chlorides used in photography are chiefly 
the salts of potassium and sodium. Both the bromides and the 
chlorides are obtained from naturally occurring salt deposits. 
but, whereas these deposits consist chiefly of chlorides, they 
contain only a very small quantity of bromide, and bromide is - 
therefore a very much more expensive material than chloride. 





The elements chlorine, bromine and iodine are ah 
atural salt or from the sea, iodine being der 


dform. Chlorine is‘a yellow- 
and poisonous, bromine gives 
more noxious than chlorine and 
ne forms shining, black crystalline 
x give a violet vapor. The chief 


and iodides used in é otography are the 





AMMONIUM CHLORIDE: Made from ammonia and hydro- 
chloric acid, should have no smell, and when evaporated by heat 
should leave no residue behind. White crystals soluble in water. 

AMMONIUM BROMIDE: Very similar to the chloride, which 
is the only impurity ly to be present. 

[UM IODIDE: Should consist of colorless crystals. 
ht and is stained yellow by the iodine liberated. 
rater and deliquescent (see p. 24). Soluble in alcohol. 


r 7 
| 














16 EASTMAN KODAK COMPANY. 


SODIUM CHLORIDE: Ordinary table salt is fairly pure sodium 
chloride and a very pure salt is easily obtained. The pure salt is 
stable and not deliquescent. Soluble in cold water to the extent of 
35%. Solubility increases very little on heating. 


SODIUM BROMIDE: Is a white salt, similar to the chloride 
but more soluble. Is generally pure but may contain chloride. 


POTASSIUM CHLORIDE: White salt, very similar to sodium 
chloride. 


POTASSIUM BROMIDE: Occurs as colorless cubical crystals 
and is generally pure. Very soluble in water. 


POTASSIUM IODIDE: Similar to bromide. Very soluble. 
May contain as impurities carbonate, sulphate and iodate, but is usually 
pure. A solution of potassium iodide dissolves iodine, which is in- 
soluble in water, and is therefore used to prepare a solution of iodine. 


The gelatin which is used to emulsify the sensitive silver 
salts is a very complex substance which is obtained from the 
bones and skins of animals, and it has some curious and valu- 
able properties. In cold water it does not dissolve but it swells 
as if, instead of the gelatin dissolving in the water, the water 
dissolves in the gelatin. If the water is heated, the gelatin 
will dissolve, and it will dissolve to any extent. It cannot be 
said that there is a definite solubility of gelatin in water in 
the same sense as salts may be considered to have a definite 
solubility. As more gelatin is added, the solution becomes 
thicker. If the gelatin solution is heated, it will become 
thinner and less viscous when hot, and will thicken again as it 
cools, but it will not recover completely. It will remain thinner 
than if it had not been heated, so that the heating of the gela- 
tin solution produces a permanent change in its pro rties. If 
a gelatin solution is cooled, t 
the solution in a dry state 


process is continued long enough, the 
set and will remain as a thick liquid. 


Gelatin belongs to the class of subs 
colloids, the name being derived from a Greek word meaning 
“oummy.’ When a gelatin jelly is dried it shrinks down 
and forms a horny or glassy layer of the gelatin itself, smooth 
and rather brittle. This dry gelatin, when placed in water, 
will at once absorb the water and swell up again to form a 
jelly. This swelling of gelatin when wet, and shrinking when 
dry, is of great importance in photography. When a photo- 






ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 17 


graphic material with an emulsion made with gelatin is placed 
in water, the film will swell up and will continue to absorb more 
water and swell for a long time, finally becoming soft and even 
dissolving, the extent to which this occurs depending on the 
temperature and the nature of the solutions in which it is 
placed. A small amount of either an acid or alkali will produce 
_a considerable increase in the swelling, and since the developer 
is alkaline and the fixing bath is acid, both these solutions have 
a great tendency to swell the gelatin, especially when they are 
warm. In order to avoid difficulty from this course, gelatin 
emulsions have a hardener added before they are coated, 
gelatin being hardened and made more resistant to swelling 
by the addition of alum. Under ordinary circumstances no 
difficulty is experienced by the photographer due to the soften- 
ing of the gelatin, but when photographic materials are ex- 
posed to extreme temperatures, care must be taken in handling 
them. Hardening agents such as alum must be added to the 
fixing bath, and all solutions must be kept at the same tempera- 
ture in order to avoid sudden contractions or expansions of 
the gelatin which may result in detaching the film from its sup- 
port or in the production of reticulation, 1. e., a coarse wrinkling 
all over the film. 





18 EASTMAN KODAK COMPANY. 


CHAPTER IIL. 
The Chemistry of Development 


When a light sensitive material is exposed for a short time 
to light, although the change which takes place may be so 
minute that it cannot be detected by any ordinary means, if the 
exposed material is placed in a chemical solution, which is 
termed the ‘‘developer,”’ the chlorine or bromine is taken away 
from the silver, and the black metallic silver which remains 
behind forms the image. This image is, of course, made up of 
grains, because the original emulsion contains the silver 
bromide in the form of microscopic crystals, and when the 
bromide is taken away from each of these, the crystal breaks 
up and a tiny coke-like mass of metallic silver remains behind 
in exactly the same position as the bromide crystal from which 
it was formed, so that, whereas the original emulsion consisted 
of microscopic crystalline grains of the sensitive silver salt, the 
final image consists of equally microscopic grains of black 
metallic silver. This removal of the bromide from the metallic 
silver is known chemically as reduction. (It must be remem- 
bered that chemical reduction has nothing to do with the photo- 
graphic operation known as the reducing of a negative, that 
is, the weakening of an over-dense negative, where the word 
simply refers to the removal of the silver and is not used in 
the chemical sense.) 


Chemical reducers are substances which have an affinity 
for oxygen and which can liberate the metals from their salts, 
such as the charcoal which, as explained in Chapter I, is used 
to reduce iron from its ore. A developing solution is therefore 
one which contains a chemical reducer. All substances which 
are easily oxidized are, however, not developers, since in order 
that a reducer may be used as the photographic developer it 
is necessary that it should be able to reduce exposed silver 
bromide but should not affect unexposed silver bromide, so that 
its affinity for oxygen must be within certain narrow bounds; 
it must be a sufficiently strong reducer to reduce the exposed 
silver salt, and at the same time must not affect that which has 
not been exposed. For practical purposes the developing 
agents are limited to a very few substances, almost all of which 
are chemically derived from benzene, the light oil which is dis- 
tilled from coal tar. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 19 


The commonest developing agents are pyrogallol (pyro), 
hydrochinon, Elon, para-aminophenol or Kodelon, and dia- 
minophenol. 


PYROGALLOL (or pyrogallic acid) is made from gallic acid, 
which is obtained from gall nuts imported from China. The gall 
nuts are fermented to obtain the gallic acid, and the gallic acid is then 
heated in a still from which the pyrogallol is distilled over. Before 
the war most of the pyrogallol used in this country was made in 
Europe, but the shortage was met by the erection of a plant by the 
Eastman Kodak Company which to-day makes all the pyrogallol 
needed for its customers. Pyrogallol is made in two forms: a flaky 
powder form and a crystal form. When the powdered pyrogallol is 
opened in the dark-room or studio, the fine particles fly about and are 
likely to settle on paper or plates, producing spots on the photographs. 
For this reason the Eastman Kodak Company supplies pyrogallol in 
the crystal form, which can be handled without any danger of par- 
ticles flying about and giving trouble. 


HYDROCHINON is made from benzene which is first converted 
into aniline and then oxidized. It is now made in several places in the 
United States, as well as by the Eastman Kodak Company. Although 
it is somewhat less powerful as a reducing agent than pyro, it gives no 
stain and when used in conjunction with Elon or Kodelon it is a very 
useful developer, in fact, it is a constituent of a majority of the better 
known commercial developers in use today. It keeps very well when 
used in tank developers because it does not oxidize as readily as pyro 
and is generally used in motion picture work. Its purity is very 1m- 
portant and Eastman Tested Hydrochinon may be relied upon for use 
in all formulas. 


Some time after pyrogallic acid and hydrochinon were in 
general use by photographers, there were introduced a number 
of new developing agents made from coal tar, which are very 
useful as supplements to the older developers. Several of these 
are based on a substance called para-aminophenol, which is 
made in the manufacture of dyes. When para-aminophenol is 
treated with methyl alcohol the methyl part of the alcohol 
attaches itself to it and forms a compound called methyl- 
para-aminophenol, which is a more active developing agent 
than the para-aminophenol itself. Another developing agent 
of the same type is diaminophenol, and is prepared in a way 
similar to para-aminophenol. 


Para-aminophenol, methyl-para-aminophenol and diamino- 
phenol are all bases and the developing agents are their salts, 
the oxalate of para-aminophenol, the hydrochloride of dia- 
minophenol being used, and the sulphate of methyl-para- 
aminophenol. 


PARA-AMINOPHENOL (OXALATE) is manufactured by the 
Eastman Kodak Company under the name of Kodelon. Many of 


20 EASTMAN KODAK COMPANY. 


the so-called “‘new’’ developing agents on the market consist entirely 
or mainly of para-aminophenol. A good sample should be light in 
color and should burn entirely when heated to redness, leaving no 
ash behind. 


MONOMETHYL PARA-AMINOPHENOL SULPHATE is manu- 
factured and sold by the Eastman Kodak Company under the name of 
Elon. Monomethy] para-aminophenol sulphate is distinguished sharply 
from para-aminophenol (oxalate) by the fact that it is soluble in the 
cold in its own weight of strong hydrochloric acid, whereas the para- 
aminophenol (oxalate) is’ insoluble. 


DIAMINOPHENOL HYDROCHLORIDE is sold by the Eastman 
Kodak Company under the trade name of Acrol. It is a steel gray 
powder, darkening easily in the air and is oxidized so rapidly in solu- 
tion that it is usual to dissolve it only when required for use. 


Different reducing agents behave differently as develop- 
ers. We cannot use Elon in the place of hydrochinon and 
get the same effect. An image developed with Elon comes up 
very quickly all over the plate and gains density slowly, while 
the hydrochinon image comes up very slowly but gains density 
steadily and rapidly. A very little change in the temperature ~ 
affects hydrochinon a good deal and affects Elon very little, and 
in the same way a small amount of sodium or potassium bro- 
mide affects hydrochinon and does not affect Elon nearly so 
much. These differences in the developing agents depend upon 
the chemical nature of the substances themselves, and the 
particular property to which these differences are due is called 
the “reduction potential” of the developer. 


The reduction potential alone does not determine the 
speed with which the developer develops the image, because 
this depends chiefly upon the rate at which the developer dif- 
fuses into the film and on the amount of developing agent and 
other substances in the developer. A high reduction potential 
enables a developer to continue to develop more nearly at a 
normal rate under adverse circumstances, such as low tempera- 
ture or the presence of bromide. The reduction potential of 
a developer, in fact, may be compared to the horse-power of 
an automobile which for other reasons than the power of its 
engine is limited in speed. If we have two automobiles and 
they are confined to a maximum speed of twenty miles an 
hour, then on level roads the one with the more powerful en- 
gine may be no faster than that with a weaker engine, but in a 
high wind or on a more hilly road the more powerful engine 
will allow the automobile to keep its speed, while the machine 
with the weaker engine will be forced to go more slowly. We 
could, indeed, measure the horse-power of an automobile by the 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 21 


maximum grade which it could climb at a uniform speed of 20 
miles an hour. 


In development, the analogy to the hill is the addition of 
bromide to the developer, since the addition of bromide greatly 
retards development, and it is found that the higher the reduc- 
tion potential of a developer, the more bromide is required to 
produce a given effect. If we measure the developing agents 
in this way, we shall find that hydrochinon has the lowest re- 
duction potential, then pyro, then Kodelon, and finally Elon, 
which has the highest. Hydrochinon has so low a potential 
that it is rarely used alone but is generally used with Elon. 
Kodelon can be substituted for Elon but more Kodelon has to 
be used in order to produce a developer of the same strength. 
Developers with a high reduction potential such as Elon, and 
to a less extent Kodelon, make the image flash up all over at 
once, because they start development very quickly even in the 
lesser exposed portions of the emulsion, while developers of 
low reduction potential, like pyro and especially hydrochinon, 
bring up the highlights of the image first and the shadows do 
not fully appear until the highlights are somewhat developed. 


Most developing agents cannot develop at all when used by 
themselves. With the exception of Acrol, developing agents, 
in order to do their work, must be in an alkaline solution, and 
the energy depends upon the amount of alkali present. The 
developers of higher reduction potential, which bring up the 
image very quickly, require less alkali than those of lower re- 
duction potential. For instance, hydrochinon is often used 
with caustic alkalis, while the other developing agents require © 
only the weaker carbonated alkali. 


The amount of alkali governs the energy of a developer, 
and if too much alkali is present, the developer will tend to pro- 
duce chemical fog, while if too little alkali is present, it will 
be slow in its action. Alkalis also soften the gelatin of the 
emulsion, and consequently too alkaline a developer will pro- 
duce over-swelling and will give trouble with frilling or blisters 
‘in warm weather. 


The alkalis used in development are of two kinds: the 
caustic alkalis and the carbonated alkalis. 


Caustic alkalis are produced when the metal itself reacts 
with water, the metals from which the alkalis generally used 
are derived being potassium and sodium. These metals are so 
easily oxidized that they have to be preserved from all contact 
with air or water by immersion in light oil or gasoline. 


22 EASTMAN KODAK COMPANY. 


If we take a small piece of sodium and place it on the sur- 
face of water in a dish, it will react with the water with great 
violence, melting with the heat produced and sputtering about 
the surface; while if we restrict its movement, the development 
of heat will be so great that the hydrogen produced will burst 
into flame. In the case of potassium, the reaction is even more 
violent than with sodium and is always accompanied by flame. 
The reaction may be represented by the equation— 


Na + H20 = NaOH oe H 
Sodium Water Caustic Soda Hydrogen 


the sodium combining with the water to form caustic soda and 
liberating hydrogen, which comes off as gas, and, as has already 
been stated, catches fire and burns in the air. This is, of 
course, not the method by which the alkalis are actually pro- 
duced. As a matter of fact, the metals are produced by elec- 
troplating the metal out from the melted alkali. 


CAUSTIC SODA is made either by the passage of an electric 
current through a solution of common salt, when the soda separates 
at one electrode and chlorine gas is liberated at the other, or from 
sodium carbonate, which is causticized by means of lime. Lime is 
calcium oxide and is prepared by heating limestone, which is calclum 
carbonate, the carbon dioxide being driven off from the limestone by 
the heat. When the lime is added to sodium carbonate, the lime 
removes the carbon dioxide from the carbonate, and leaves the sodium 
hydrate in the solution, which is then evaporated to get the solid sub- 
stance. At present, caustic soda is easily obtained in a very pure state, 
and there is usually no difficulty in getting good caustic soda for photo- 
graphic work. It must be protected from the air, since it easily absorbs 
moisture and carbon dioxide. As its name indicates, it is very caustic 
and attacks the skin, clothing, etc. 


CAUSTIC POTASH is very similar to caustic soda and is pre- 
pared in the same way. Fifty-six parts of caustic potash are chemi- 
cally equivalent to forty parts of caustic soda. 


An alkali which was often used with pyrogallol in the early 
days of photography, but which is rarely used nowadays, is 
ammonia. Nitrogen combines with three times its volume of 
hydrogen to form a gas, NH3. This gas is known as ammonia 
and is very soluble in water, its solution being strongly alkaline. 
Ammonia combines directly with acids to form salts which are 
analogous to the salts of sodium and potassium. Thus with 
hydrochloric acid it forms ammonium chloride, which is similar 
to sodium chloride and potassium chloride: 


NHs = HCl = NH.Cl 
Ammonia Hydrochloric Acid Ammonium Chloride 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 23 


Ammonia is a somewhat weaker alkali than soda or potash but 
stronger than the carbonates. For use in development it has 
the disadvantage that being used in the form of a solution of 
a gas its strength is somewhat uncertain and variable, the am- 
monia escaping from the solution. Also, it is a solvent of silver 
bromide and tends to produce colored fogs which are not so 
easily produced with other alkalis. 


AMMONIA SOLUTION is commercially prepared from the am- 
moniacal liquor obtained in the distillation of coal for coal gas. The 
liquor is neutralized with sulphuric acid, the ammonium sulphate 
crystallized out, and the ammonia gas liberated from the sulphate 
with lime and led into water, in which it dissolves. The solution is 
usually free from impurities. 


Ammonia solutions are prepared Ee aa in two strengths 
“ammonia water,’”’ containing 10% of ammonia gas by weight bad 
having a specific ‘gravity of .96, and ‘‘stronger ammonia water’ con- 
taining 28% of ammonia by weight and having a specific gravity of .90. 


The alkalis generally used for photographic work are not 
the caustic alkalis but the carbonates, which are salts of car- 
bonic acid, H2CO3. Carbonic acid is a very weak acid, so that 
in solution the carbonates are not neutral but alkaline because 
of the predominance of the strong base over the weak acid, the 
carbonate being, to some extent, split up into the bicarbonate 
or acid carbonate and the caustic alkali. The use of a carbon- 
ate in development therefore represents a sort of reservoir of 
alkali, only a small amount of alkali being present at any time, 
but more being generated by dissociation of the carbonate as 
itis used up. If instead of using carbonate we were to use for 
development a solution containing a proportional amount of 
caustic alkali, we should have only a small amount of alkali 
present, and it would soon be exhausted. The use of carbonate, 
therefore, enables us to employ a small concentration of alkali 
and yet to keep that concentration nearly constant during use. 


When a salt is dissolved in water at a high temperature un- 
til no more will dissolve and then the solution is allowed to 
cool, the salt will generally be deposited in crystals; sometimes, 
as in the case of silver nitrate, the crystals consist of the pure 
substance, but more often each part of the salt combines with 
one or more parts of water to form the crystals. This com- 
bined water is called “‘water of crystallization.”’ Thus, crystals 
of sodium carbonate formed from a cool solution contain ten 
parts of water to one of aroon ales and their composition 
should be written: 


NazCO2 e 10H20 


24 EASTMAN KODAK COMPANY. 


What is called in the last paragraph a “‘part”’ of sodium car- 
bonate, NazCOs, will weigh 106 units, while a ‘‘part’”’ of water, 
H.0, weighs 18 units, so that the crystals of sodium carbonate 
contain 106 parts by weight of sodium carbonate and 180 parts 
by weight of water, and consequently crystallized sodium car- 
bonate contains only 37% of dry sodium carbonate. If sodium 
carbonate is crystallized from a hot solution only one part of 
water is combined in the crystals with each part of sodium 
carbonate so that they have the composition NazCO; . H2O 
and contain 85% of dry carbonate. Sodium carbonate con- 
taining ten parts of water of crystallization loses nine of them 
by drying in the air and breaks up, forming the compound with 
one part of water. This last part of water is only removed 
with difficulty by heating in the air, when the dry carbonate is 
formed, containing only a small residual amount of water and 
about 98% carbonate. 


When exposed to the air chemicals often either absorb or 
give up water. Those which absorb water are said to be “hy- 
groscopic,”’ and if they absorb so much that they dissolve and 
form a solution they are said to be ‘‘deliquescent.’”’ Chemicals 
which give up water to the air, so that the crystals break down 
and become covered with powder, are called “efflorescent.” 


SODIUM CARBONATE comes on the market in three forms: 
Crystals with ten parts of water, NazCO3. 10H2O containing 37% of 
the carbonate; crystals with one part of water, NazCO3.H2O0, con- 
taining 85% of the carbonate, and the dry powder containing 98% of 
the carbonate. The carbonate is made by treatment of salt solution 
with ammonia and carbon dioxide which reacts with the salt to produce 
sodium bicarbonate, NaHCO3. The bicarbonate is heated and half 
of the carbonic acid is driven off, producing crude sodium carbonate, 
which at this stage is known as “‘soda ash.” This is then dissolved in 
water, and crystals of ‘‘sal soda,”’ containing ten parts of water, are pro- 
duced. From this a crystalline salt with either one or ten parts of 
water is prepared for photographic use, but owing to the uncertainty 
of the composition of these crystals it is better to prepare the pure 
dry carbonate. This is obtained by heating the pure bicarbonate 
which can be precipitated from a solution of sal soda by means of ear- 
bon dioxide gas. When the bicarbonate is heated in the air, half of 
the carbonic acid is driven off, and sodium carbonate, NazCOs, is pro- 
duced according to the equation: 


SNaHCOs ot see 4. as Paired 
Sodium Bicarbonate Sodium Carbonate Carbon Dioxide Water 


The exact amount of heating is very important. If it is not done for 
sufficient time there will be a large amount of bicarbonate left in the 
product, and bicarbonate is practically useless as an alkali in photog- 
raphy. On the other hand, if heating is continued too long, caustic 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 25 


soda will be produced. In the preparation of photographic carbonate 
the heating should be continued so that the material is almost pure 
sodium carbonate containing practically no bicarbonate but is very 
slightly on the alkaline side. Much caustic soda would be fatal, but it 
is better to have a trace of caustic soda than bicarbonate. The prepar- 
ation of carbonate of soda is a matter to which the greatest attention 
is given by the Eastman Kodak Company, and the E. K. Tested Car- 
bonate is specially prepared to meet the needs of the photographer. 


POTASSIUM CARBONATE is sometimes substituted for sodium 
carbonate in developer formulas. Although it is more soluble and is 
a somewhat stronger alkali than sodium carbonate, it has the dis- 
advantages of being more expensive and absorbs water very readily. 
It must, therefore, be kept in well-sealed bottles. 


Owing to the fact that developers are necessarily substances 
which have a great affinity for oxygen and that the air contains 
oxygen, developing solutions containing only the developing 
agent and alkali would be rapidly spoiled from oxidation by 
the air. In order to make the developer keep there is added to 
the developing solution, in addition to the reducing agent and 
alkali, some sulphite of soda. Sulphite of soda has a very 
strong affinity for oxygen, being easily oxidized to sulphate of 
soda (see page 11), so that it protects the developer from the 
oxygen of the air, thus acting as a “‘preservative.”’ This action 
of the sulphite is very easily seen with the pyrogallol developer. 
The oxidation product of pyrogallol is yellow, and this oxida- 
tion product which is formed in development is deposited in the 
film along with the silver, so that if we use a pyrogallol devel- 
oper without sulphite we shall get a very yellow negative, the 
image consisting partly of silver and partly of the oxidized 
pyrogallol. If we use sulphite in the developer, the image will 
be much less yellow because the pyrogallol will be prevented 
from oxidizing, the sulphite being oxidized instead, and finally 
if we add a great deal of sulphite, we shall get almost as blue 
an image as with Elon, the oxidation product of which is not 
deposited in a colored form with the silver. 


SODIUM SULPHITE is prepared by blowing sulphur dioxide 

as into a solution of carbonate of soda. When sulphite is crystalized 
rom the cooled solution it forms crystals containing seven parts 
of water to one of sulphite, of the composition NazSO3 . 7H20 which 
contain, when pure, 50% of dry sulphite. These crystals give up 
water when kept in the air and form a white powder on the surface. 
Since sulphite, when exposed to the air, has a tendency to oxidize to 
the sulphate, and as the sulphate is not a preservative, it is well 
to view with suspicion sulphite which has effloresced to a great 
extent. A quick rinse in cold water will remove the white powder 
from the crystals. 


Sulphite free from water is produced by two methods: by drying 
the crystals, which produces what is called the “‘desiccated”’ salt, 


26 EASTMAN KODAK COMPANY. 


containing about 92% of pure sulphite, and by precipitation from hot 
solutions which gives a compound generally called ‘‘anhydrous” 
sulphite, and which contains as much as 96.5% of sulphite. 


_ Eastman Tested Sulphite is the desiccated salt, and is prepared 
in a very pure state almost free from sulphate. If If prepared in this 
aay as a dry powder the sulphite will keep well for a long time. 


Seis forms a number of compounds with sulphurous 
acid in addition to sodium sulphite itself. Thus we have 
sodium acid sulphite or bisulphite, NaHSO:, which may be 
regarded as a compound of sodium sulphite with sulphurous 
acid: 


Na2SO3 +. H2SO3 = 2N aHSOs3 
Sodium Sulphite Sulphurous Acid Bisulphite of Soda 


Again we have sodium metabisulphite, which is a compound of 
sodium sulphite with sulphur dioxide: 


Naz2S205 


Na2SOz + SO2 
Sodium Sulphite Sulphur Dioxide Saat Metabisulphite 


These acid sulphites are very similar in their properties and 
probably form the same solution when dissolved in water. 


POTASSIUM METABISULPHITE is often used as a preserv- 
ative. It forms good crystals and is convenient in use but is very 
costly in comparison with sodium bisulphite. 


SODIUM BISULPHITH, when pure, is a white salt which has 
an acid reaction, often containing a slight excess of sulphur dioxide. 
Since sodium sulphite is an alkaline salt, owing to the predomi- 
nance of the strong base, soda, over the weak sulphurous acid, a neu- 
tral solution can be produced by adding a small amount of bisulphite 
to sulphite, and this neutral solution has found extensive application 
as a preservative for a pyro developer. Bisulphite is used very 
largely as a preservative for fixing baths, supplying both the sulphite 
and the acid necessary. 


It is difficult to prepare bisulphite free from iron, and any iron 
in the bisulphite produces a dark color when used for making up a 
pyro solution. 


The Eastman Kodak Company has an entirely satisfactory 
bisulphite and lists it among its Tested Chemicals. 


It is often customary to substitute sodium bisulphite for 
potassium metabisulphite weight for weight. It really sim- 
mers down to a matter of dollars and cents because either 
chemical is quite satisfactory for the purpose but, as a rule, 
sodium bisulphite ranges in cost from 4 to \% that of potas- 
sium metabisulphite. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 27 


Since sodium bisulphite may be considered as a compound 
of sodium sulphite and sulphurous acid, while sodium sulphite 
is alkaline, bisulphite is preferable as a preservative in the case 
of a two-solution developer, since oxidation progresses less 
readily in acid than in alkaline solution. 

In the case of a one-solution developer containing, say, 
sodium sulphite, sodium bisulphite and sodium carbonate, the 
bisulphite is converted to sulphite by the sodium carbonate 
according to the following equation: 


Sodium Bisulphite + Sodium Carbonate = Sodium Sulphite + Sodium Bicarbonate 


so that a corresponding amount of sodium sulphite might just 
as well have been added in the first place. Sodium bisulphite 
also neutralizes or destroys an equivalent amount of sodium 
carbonate, thus reducing the proportion of alkali and therefore 
exerts an apparent restraining action, while the developer ap- 
parently keeps longer because some of the carbonate has been 
destroyed. 


28 EASTMAN KODAK COMPANY. 


CHAPTER IV. 


The Chemistry of Fixation 


After development, the undeveloped silver bromide is re- 
moved by immersion of the negative or print in what is called 
the “‘fixing’”’ bath. There are only a few substances which will 
dissolve silver bromide, and the one which is universally used 
in modern photography is sodium thiosulphate, NazS2Os, 
which is known to photographers as hyposulphite of soda, or 
more usually as hypo, though the name hyposulphite of soda 
is used by chemists for another substance. 


THIOSULPHATE OF SODA or HYPO can be made by boiling 
together sodium sulphite and sulphur, the sulphur combining with 
the sodium sulphite according to the equation 


Naz2SO3 7 S = Na2S203 
Sodium Sulphite Sulphur Hypo 


In practice it is generally made from calcium sulphite residues, the 
calcium thiosulphate being then converted into the sodium salt by 
treatment with sodium sulphate. The hypo comes on the market 
in clear crystals and is usually fairly pure, any foreign substance 
present being more often due to accidental contamination than of a 
chemical nature and consisting of dirt, straw or wood dust due to 
careless handling. Sometimes, however, the hypo contains calcium 
thiosulphate, which decomposes much more readily than the sodium 
salt. On the whole, it is not difficult to obtain good hypo; the East- 
man Tested Hypo is prepared in the form of granular crystals, easy 
to dissolve, and free from accidental contamination. 


In the process of fixation the silver bromide is dissolved in 
the hypo by combining with it to form a compound sodium 
silver thiosulphate. Two of these compound thiosulphates 
exists, one of them being almost insoluble in water, while the 
other is very soluble. As long as the fixing bath has any ap- 
preciable fixing power the soluble compound only is formed. 


Fixing is accomplished by means of hypo only, but mate- 
rials are usually transferred from the developer to the fixing 
bath with very little rinsing so that a good deal of developer is 
carried over into the fixing bath, and this soon oxidizes in the 
bath, turning it brown, and staining negatives or prints. In 
order to avoid this the bath has sulphite of soda added to it 
as a preservative against oxidation, and the preservative action 
is, of course, greater if the bath is kept in a slightly acid state. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 29 


In order to prevent the gelatin from swelling and softening 
it is also usual to add some hardening agent to the fixing bath 
so that a fixing bath instead of containing only hypo will con- 
tain in addition sulphite, acid, and hardener. 

Now, if a few drops of acid, such as sulphuric or hydro- 
chloric acid are added to a weak solution of hypo, the hypo 
will be decomposed and the solution will become milky, owing 
to the precipitation of sulphur. This is because the acid con- 
verts the sodium thiosulphate into the free thiosulphuric acid, 
and this substance is quite unstable, decomposing into sul- 
phurous acid and sulphur according to the equation: 


H28203 = 


H2S8O3 + S 
Thiosulphuric Acid Sulphurous Acid Sulphur 


The change of thiosulphate into sulphite and sulphur is rever- 
sible, since, if we boil together sulphite and sulphur we shall 
get thiosulphate formed, so that while acids free sulphur from 
the hypo, sulphite combines with the sulphur to form hypo 
again. Consequently, we can prevent acid decomposing the 
hypo if we have enough sulphite present, since the sulphite 
works in the opposite direction to the acid. An acid fixing bath, 
therefore, is preserved from decomposition by the sulphite, 
which also serves to prevent the oxidation of developer carried 
over into it. The developer which is carried over into the 
fixing bath is, however, alkaline and consequently a consider- 
able amount of acid is required in a fixing bath which is used 
for any length of time, since if only a small amount is present, 
it will soon be neutralized by the developer carried over. We 
are, therefore, in the difficult position that we require a large 
amount of acid present, and yet the fixing bath must not be 
strongly acid. The solution of the difficulty is found by taking 
advantage of the fact that there are some acids which are very 
weak in their acidity and yet can neutralize alkali in the same 
way as a strong acid, so that a large amount of these acids can 
be added without making the bath so acid that sulphur is pre- 
cipitated. 


The strength of an acid depends upon the fact that when 
it is dissolved in water some of the hydrogen contained in it 
dissociates from the acid and remains in the solution in an 
active form, and the acidity of the solution depends upon the 
proportion of the hydrogen which is dissociated into this active 
form. The amount of alkali which the acid can neutralize, 
however, depends upon the total amount of the hydrogen pres- 
ent, and not on the dissociated portions only. The strongest 


30 EASTMAN KODAK COMPANY. 


acids are the mineral acids, such as sulphuric and hydrochloric 
while the weakest acids are the organic acids, such as citric 
and acetic acids. 


Since a large amount of a weak acid is required, the best 
acid for the purpose is acetic acid. 


ACETIC ACID is prepared by the fermentation of apple juice, 
yielding a product commonly called vinegar. In addition to acetic 
acid, vinegar also contains many impurities and the acid strength 
is from 4% to 8%. The stronger acid is made from acetate of lime 
which is prepared either by neutralizing vinegar with chalk or, more 
commonly, by neutralizing with lime the crude acetic acid prepared 
by the destructive distillation of wood. Acid thus prepared may 
contain as high as 99.5% acetic acid and is usually called glacial 
acetic acid because, at moderately low temperatures, it freezes to a 
solid. Dilutions of the glacial acid are commonly supplied contain- 
ing 80% and 28% acetic acid. This 28% acetic acid, prepared by 
diluting the pure glacial acetic acid, must not be confused with 
“commercial 28% acetic acid”’ which is prepared by redistilling the 
acid obtained by the destructive distillation of wood and contains 
many impurities which have a decidedly deleterious effect on photo- 
graphic materials. 


When acetic cannot be obtained for the fixing bath, the 
only substitute which appears to be generally available is 
sodium bisulphite. Bisulphite of soda, NaHSOs, is intermediate 
between sulphite of soda and sulphurous acid, and is, therefore, 
equal in acidity to a mixture of equal proportions of these two 
substances. It makes a satisfactory acid fixing bath but does 
not give quite as good a reserve of available acid in the bath as 
acetic acid does. This is of importance particularly in connec- 
tion with the hardening agent used in the fixing bath. 


The commonest hardening agent is potash alum, the alums 
having the property of tanning gelatin. 


ALUM is a compound sulphate of sodium, potassium or am- 
monium with aluminum. If the hydrogen in sulphuric acid be re- 
placed by potassium, we get potassium sulphate, K2SO., while if it 
be replaced by aluminum, we get aluminum sulphate, Ale (SOx)s. 
The aluminum sulphate combines with other sulphates to form the 
alums, of which the commonest are potassium alum and ammonium 
alum. Sodium alum does not crystallize well, but the potassium 
and ammonium salts crystallize in large, clear crystals, and are 
convenient in use. 


POTASSIUM CHROME ALUM, which is often used in the 
place of ordinary alum, does not contain any aluminum in spite of 
its name. It is a compound sulphate of potassium sulphate with 
chromium sulphate, of which the formula is Cre(SO.)3, the chromium 
taking the place of the aluminum present in aluminum sulphate. 
Chrome alum is prepared commercially in large quantities and of 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 31 


a high degree of purity. It occurs as violet crystals soluble in water, 
its solution in cold water being violet but going green on heating 
owing to the change in the composition of the salt. 


AMMONIUM CHROME ALUM is chemically very similar to 
potassium chrome alum except that it is the compound of ammo- 
nium sulphate with chromium sulphate. Despite its very close chem- 
ical similarity to the potassium compound, it is found that it has a 
decided tendency to produce fog, even though the fixing bath is 

{maintained acid. Just why this is has not been satisfactorily ex- 
plained. However, this warning is given because at times, due to 
price fluctuations, the used ammonium alum is sometimes slightly 
advantageous from the point of view of dollars and cents, but it is 
always safer to use a potassium salt even though it may cost one or 

, two cents a pound more. 


In the presence of sodium sulphite a solution of chrome 
alum loses its hardening properties somewhat rapidly, depend- 
ing upon the concentration of the chrome alum and the sodium 
sulphite. A fresh chrome alum fixing bath containing hypo, 
chrome alum, and sodium bisulphite loses its hardening prop- 
erties in the course of one or two days even if the bath is not 
used. A chrome alum fixing bath containing from 1 to 2% 
chrome alum is only useful when used immediately after 
preparation, although a bath containing from 5 to 10 per cent 
chrome alum will maintain its hardening properties for two or 
three days. Chrome alum is most useful as a hardening bath 
between developing and fixing. A plain solution of chrome 
alum retains its hardening properties indefinitely, though with 
use when developer is carried over by the plates and films, the 
hardening properties of the bath fall off owing to the presence 
of sodium sulphite in the developer. 


FORMALIN is a solution of formaldehyde, a gas having a very 
strong odor. The commercial solution contains 40% of formalde- 
hyde and has the property of hardening gelatin very powerfully, a 
5% solution rendering the gelatin of a film completely insoluble in 
boiling water in less than a minute. It should be used only in alkaline 
or neutral solutions because in acid solutions it does not harden. 
Formalin, however, irritates the mucous membrane of the nose and 
throat and is very objectionable to certain individuals. 


It is important not to overwork a fixing bath, because as 
the fixing bath becomes saturated with silver the film or paper 
will carry this silver into the wash water with it and if not 
properly washed the silver salt will remain in the finished pho- 
tograph and will decompose into silver sulphide in time, pro- 
ducing stains. A gallon of the standard strength fixing bath 
will fix one hundred 8 x 10 prints, and when these have been 
fixed the bath should be changed. 


32 EASTMAN KODAK COMPANY. 


CHAPTER V. 


The Chemistry of Toning 


The operation of toning consists in the deposition on the 
silver image of another substance having a different color, in 
order to get a more pleasing result, or of the transformation 
of the silver image into another substance for the same purpose. 

There are four principal methods of toning: 


A. Toning by the replacement of the silver by other 
metals; 
B. Toning by the deposition of salts of metals; 


C. Toning by the transformation of the silver image into 
some substance to which dyes will attach themselves 
in an insoluble form; 


D. Transformation of the silver image into a stable, 
strongly colored salt of silver. 


A. In the case of prints which are made by the printing- 
out processes, the silver compound produced by the action of 
light is colored, and after fixation the image left is usually of 
an unpleasant color,—a yellow or yellow-brown—and in order 
to change this to a more satisfactory color it is toned by means 
of gold or, more rarely, platinum. 


When a silver image is placed in a solution of gold or 
platinum the silver will replace the metal in solution, going 
into solution itself, and the gold or platinum will be deposited 
in the place of the silver. The rate at which these metals are 
deposited is very important, especially in the case of gold 
toning. If the gold is deposited too slowly, it will be deposited 
in a very fine condition, and in the case of finely divided metals, 
their color depends upon the fineness of the division. Finely 
divided gold is red, which is not as pleasing as the blue gold 
obtained by more rapid deposition. 


In order to ensure rapid deposition it is necessary that the 
bath should be kept alkaline, and consequently borax or sodium 
acetate is added to the gold chloride to make a toning bath, 
while sometimes substances having a weak reducing action are 
added, such as sulphocyanides or formates. Platinum toning 
baths are used in an acid condition. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 33 


The chemicals used for making up these toning baths must 


be of high purity, and it is best to get tested chemicals in all 
cases. 


GOLD CHLORIDE is made by dissolving gold in a mixture of 
hydrochloric and nitric acids and evaporating the solution. It forms 
brownish crystals, rapidly absorbing water, which contain 65% 
metallic gold. The salt is sold in small glass tubes containing 15 
grains, and in order to use it, the label is removed from the tube and 
the tube is broken in a bottle containing a known amount of water 
so that a solution of definite strength is obtained without danger 
of losing the precious material. 


GOLD SODIUM CHLORIDE is a double chloride of gold and 
sodium which occurs in yellow crystals and contains 49% of metallic 
gold. It has the advantage over the pure chloride of gold that itis 
neither acid nor deliquescent. 


POTASSIUM CHLOROPLATINITE is the double chloride of 
platinum and potassium, and is the form in which platinum is 
used for a toning bath. It occurs in reddish crystals, and is supplied 
in sealed glass tubes like gold chloride. 


LEAD NITRATE and LEAD ACETATE. These colorless 
salts of lead are sometimes used for toning baths. They are both 
soluble in water and the solutions are very poisonous. 


SODIUM ACETATE, SODIUM PHOSPHATE and BORAX 
are all weak alkalis and are used in gold toning baths for this reason. 
They occur as white salts, soluble in water. Borax occurs as a 
mineral and is largely used in industry. Only the pure salt should 
be used for photographic purposes. 


AMMONIUM SULPHOCYANATE, SULPHOCYANIDE or 
THIOCYANATE, is a salt occurring in very deliquescent crystals. 
In order to be at all certain of its strength it must be preserved with 
great care, out of contact with the air. It is one of the most popular 
salts for use with gold chloride in toning baths. 


B. A good many metallic compounds are colored, and if 
the silver image is replaced by these colored compounds, wholly 
or in part, a colored image is obtained. In most of the toning 
processes based upon the use of colored compounds, ferro- 
cyanides of metals are employed, the silver image being first 
transformed into silver ferrocyanide, the silver in the silver 
ferrocyanide being then substituted by another metal of which 
the ferrocyanide is colored. 


The ferro- and ferricyanides are very complex compounds. 
The cyanides themselves are compounds containing carbon 
and nitrogen, and have a curious resemblance to chlorides and 
bromides. Hydrogen unites with carbon and nitrogen to form 
an acid, HCN, which is called hydrocyanic acid, and which is 
known popularly as prussic acid. The hydrogen in this can be 
substituted by metals to form cyanides such as potassium 


34 EASTMAN KODAK COMPANY. 


cyanide, KCN, which is analogous to potassium chloride, KCl, 
or potassium bromide, KBr, and on adding a solution of silver 
nitrate to a soluble cyanide, silver cyanide, AgCN, is precip- 
itated as an insoluble salt, just as silver chloride or silver 
bromide is precipitated. 


There is one respect, however, in which hydrocyanic acid 
and the cyanides differ from the corresponding chlorine or 
bromine compounds, and this is that they are extremely poi- 
sonous. A trace of cyanide swallowed will cause death. 


Cyanide solutions are solvents for the silver halides, form- 
ing soluble double compounds with the insoluble silver salts. 
Potassium cyanide is employed for fixing wet collodion plates, 
which, being made from silver iodide, are not easily fixed in 
hypo. Whenever cyanides are used by photographers, their 
extremely poisonous nature should be remembered and every 
possible care taken in keeping and using them. 


The cyanides easily form complicated double compounds. 
With sulphur, for instance, they form sulphocyanides, and 
ammonium sulphocyanide has already been referred to as 
being used in gold toning baths. The cyanides unite with iron 
cyanides to form two important groups of compounds called 
ferrocyanides and ferricyanides. These differ from each other 
in their degree of oxidation, the ferricyanides being more 
highly oxidized than the ferrocyanides, so that when a ferri- 
cyanide is reduced a ferrocyanide is formed. 


POTASSIUM FERROCYANIDE is yellow. It is known as 
ve ey prussiate of potash” and has very_little application in photog- 
raphy. 

POTASSIUM FERRICYANIDE or RED PRUSSIATE OF 
POTASH is prepared by passing chlorine gas into a solution of the 
ferrocyanide and is deposited from concentrated solution as red 
crystals. The crystals are soluble in water to a yellow solution which 
does not keep well. 

The value of ferricyanide in photography lies in the fact 
that ferricyanide oxidizes the silver image and forms silver 
ferrocyanide from it, so that if a negative is placed in a solu- 
tion of ferricyanide, it is slowly bleached to silver ferrocyanide. 


This property can be made use of in various ways. The 
silver ferrocyanide is soluble in hypo so that if we use a solu- 
tion of potassium ferricyanide and hypo instead of plain 
potassium ferricyanide, we shall not get a white image produced 
but the silver image will be slowly dissolved, since it will be 
converted into the silver ferrocyanide by the ferricyanide and 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 35 


then the silver compound formed will be dissolved in the hypo. 
This mixture of ferricyanide and hypo is known as Farmer’s 
Reducer, and will be referred to in the next chapter. Again, 
if we add bromide to our ferricyanide solution, silver bromide 
is more insoluble than silver ferrocyanide and consequently 
the silver ferrocyanide as it is produced will be transformed 
into silver bromide. This operation of transforming a silver 
image into a bromide image is generally known as bleaching. 
If we combine with the potassium ferricyanide a salt of a metal 
which gives an insoluble colored ferrocyanide, then we shall 
get the silver ferrocyanide formed, and this will be converted 
into the ferrocyanide of the metal whose salt has been added 
to the bath. If we add an iron salt, such for instance as iron 
citrate, to the potassium ferricyanide, we shall get a blue iron 
ferrocyanide formed and the image will be toned blue. If we 
use uranium nitrate, we shall get the reddish brown uranium 
ferrocyanide, while if we use copper citrate, we shall get the 
red copper ferrocyanide. Sometimes instead of using the metal 
salt in the same bath as the ferricyanide the operation is done 
in two steps, the silver being first bleached to silver ferro- 
cyanide, and this being then combined with a salt of the metal 
to form the colored metallic ferrocyanide. 


C. The range of colors which can be obtained by the use 
of colored metals or metallic compounds is rather limited, and 
in order to get a wider range, especially for motion picture and 
lantern slide work, experimenters have tried to find methods 
of using dyes and attaching them to the image. 


It has been found that this can be done by transforming 
the silver image into silver iodide, which can be accomplished, 
for instance, by treatment of the image with a mixture of 
potassium ferricyanide and potassium iodide. The silver 
iodide image formed in this way will mordant basic dyes and 
attach them to the image so that the image assumes the color 
of the dye. The Eastman Kodak Company has recently 
worked out a new process in which instead of transforming 
the silver image into silver iodide it is treated with a copper 
toning bath and transformed into copper ferrocyanide, and 
then the basic dyes are mordanted on to the copper ferro- 
cyanide image. The copper ferrocyanide is red; but very little 
is required to mordant a good deal of dye and the image is 
very transparent, so that this new process makes it possible 
to obtain very good results by dye toning. Full particulars of 
this process are given in our booklets on lantern slide making 
and on the toning of motion picture film. 


36 KASTMAN KODAK COMPANY. 


D. Silver sulphide is a very insoluble compound of 
silver, and consequently if a silver image or a silver halide 
salt is treated with sulphur or a sulphide respectively they will 
at once be transformed into silver sulphide. Silver sulphide 
has a color varying from light brown to black, according to its 
state of subdivision, and the transformation of the image into 
silver sulphide is by far the most popular method of toning 
developing-out paper prints, the prints so toned being generally 
known as “‘sepia’”’ prints. 

There are two general methods of transforming the image 
into silver sulphide: 


A. Direct toning, with the hypo alum bath; and 
B. Bleaching and redevelopment. 


A. As was explained in the chapter dealing with fixing, 
when an acid is added to a solution of hypo, it tends to precip- 
itate sulphur. Now, asolution of alum in water is weakly acid, 
so that if alum is added to plain hypo without any sulphite 
present, the solution will, after a time, become turbid and pre- 
cipitate sulphur. This solution of alum and hypo at the point 
where it is ready to precipitate the sulphur may be considered 
as having free sulphur in solution, and if prints are immersed 
in a hot solution of alum and hypo, the silver image will be 
converted directly into silver sulphide and the prints will be 
toned brown. Only one precaution is necessary in order to 
obtain successful results with the hypo-alum toning bath. The 
bath tends to dissolve the image and consequently if a fresh 
bath is used, it will weaken the print, eating out the highlights. 
In order to prevent this a little silver must be added to the bath 
either in the form of silver nitrate or by toning a number of 
waste prints or by throwing in old Solio prints, which contain 
free silver. A bath lasts for a long time, and as a general rule 
a hypo-alum bath which has been somewhat used works better 
than a fresh bath. 


B. The greatest objection to the hypo-alum bath is that 
the bath has a somewhat disagreeable odor, sulphur compounds 
being liberated from it, and it is rather troublesome to use a 
bath which has to be heated, so that while hypo-alum toning 
is used on the large scale, smaller quantities of prints are com- 
monly toned by bleaching the silver bromide print in a bath of 
ferricyanide and bromide, and then treating the bleached print 
after washing, with sodium sulphide, which converts the silver 
bromide directly into silver sulphide. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 37 


SODIUM SULPHIDE occurs in white, transparent crystals, 
which have a strong affinity for water and quickly deliquesce unless 
kept carefully protected from the air. It is best kept in a strong 
stock solution. So much trouble has been caused by impure sodium 
sulphide that of recent years the Eastman Kodak Company has 
been supplying a sulphide which they have fused so that it will 
contain no moisture and be of definite purity. One part, by weight, 
of the fused sulphide is equivalent to three parts, by weight, (approx.) 
of the crystals. 


Sodium sulphide often contains impurities, chiefly iron, though 
by dissolving in hot water the iron sulphide quickly separates out 
as a black sludge, leaving a clear solution which should be decanted. 
Old sodium sulphide often contains hypo, since hypo is produced in 
the oxidation of sulphide, and if hypo is present in any considerable 
amount, some of the silver bromide will be dissolved by it and the 
sais will lose strength in the highlights and give a very inferior 
result. 


All sulphides give off a certain amount of hydrogen sul- 
phide, which smells offensively, and which is extremely dan- 
gerous to photographic materials, since a very small amount 
of hydrogen sulphide will convert enough of the silver bro- 
mide or chloride of the material into sulphide to produce a 
severe fog. No photographic materials should therefore be 
stored in a room where sulphides are kept or where sulphide 
toning is done. 


It has already been explained that the color of silver sul- 
phide depends upon its state of division, and since the state of 
division of the toned image depends upon that of the untoned 
image and this again upon the treatment of the material, it is 
evident that the exposure and development of the print will 
have an effect upon the result obtained. As a general rule, it 
may be stated that to get good colors in sulphide toning it is 
necessary that a print should have been fully developed and 
not over-exposed; a print which is very fully exposed and then 
developed for a short time will not give a good tone. 


38 EASTMAN KODAK COMPANY. 


The Chemistry of Reduction and 


Intensification 


REDUCTION. 


By reduction in photography is meant the removal of some 
silver from the image so as to produce a less intense image. 
Thus, in the case of an over-developed plate there will be too 
much density and contrast, and the negative may be reduced to 
lessen this. In the case of an over-exposed negative there may 
not be an excess of contrast but the negative will be too dense 
all over, and in this case what is required is the removal of 
the excess density. 


It is unfortunate that the word “reduction” is used in Eng- 
lish for this process. In other languages the word ‘‘weakening”’ 
is used, and this is undoubtedly a better word, because the 
chemical action involved in the removal of silver from a nega- 
tive is oxidation, and the use of the word reduction leads to 
confusion with true chemical reduction, such as occurs in de- 
velopment. 


All the photographic reducers are oxidizing agents, and 
almost any strong oxidizing agent will act as a photographic 
reducer and will remove silver, but various oxidizing agents 
behave differently in respect to the highlights and shadows of 
is image. Reducing solutions can be classified in three 
classes: 


a. Cutting reducers 
b. True scale reducers 
c. Flattening reducers. 


A. The cutting reducers remove an equal amount of sil- 
ver from all parts of the image and consequently remove a 
larger proportion of the image from the shadows than from 
the highlights of the negative. The typical cutting reducer is 
that known as Farmer’s Reducer, consisting of a mixture of 
potassium ferricyanide and hypo, the potassium ferricyanide 
oxidizing the silver to silver ferrocyanide and the hypo dis- 
solving the latter compound. Farmer’s Reducer will not keep 
when mixed, decomposing rapidly, so that it is usually made 
by making a strong solution of the ferricyanide and then add- 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 39 


ing a few drops of this to a hypo solution when the reducer is 
required. It is especially useful for clearing negatives or lan- 
tern slides which show slight fog, and is also used for local 
reduction, the solution being applied with a brush or a wad of 
absorbent cotton. 


Another cutting reducer is permanganate. The perman- 
ganates are very strong oxidizing agents, and if a solution of 
permanganate containing sulphuric acid is applied to a nega- 
tive, it will oxidize the silver to silver sulphate, which is 
sufficiently soluble in water to be dissolved. 


Permanganate has only a very weak action on a negative 
if no acid is present and this may be made use of for the 
removal of ‘‘dichroic”’ fog, the yellow or pink stain sometimes 
produced in development. Dichroic fog consists of very finely 
divided silver and this is attacked by a solution of plain per- 
manganate which will have no appreciable action on the silver . 
of the image. 


An important difference between the behavior of ferricyan- 
ide and permanganate when used for reducing pyro-developed 
negatives should be noted. In a negative developed with pyro 
the image consists partly of the oxidation product of the pyro 
associated with the silver. (See p. 25). When such a negative 
is reduced with ferricyanide the silver is removed but the stain 
is unattacked so that the negative appears to become yellower 
during reduction, though the ferricyanide does not really pro- 
duce the color, only making it evident by removal of the silver. 
Permanganate, on the other hand, attacks the stain image in 
preference to the silver and consequently makes the negative 
less yellow. Permanganate can also be used as an alternative 
to ferricyanide for bleaching negatives, since if bromide be 
added to the solution silver bromide will be formed and the 
same bleaching action obtained as with ferricyanide. 


POTASSIUM PERMANGANATE occurs in dark purple 
crystals which dissolve to form a purple solution. It is easily ob- 
tained pure but there is a good deal of impure permanganate on the 
market; Eastman Tested Permanganate is a pure product. 


In addition to its use for reduction and bleaching, perman- 
ganate is employed as a test for hypo, since it is at once reduced 
by hypo, and the colored solution of the permanganate, there- 
fore, loses its color in the presence of any hypo. It may con- 
sequently be used to test the elimination of hypo from negatives 
or prints in washing. When permanganate is reduced in the 
absence of an excess of free acid, a brownish precipitate of 


40 EASTMAN KODAK COMPANY. 


manganese dioxide is obtained and sometimes negatives or 
prints which have been treated with permanganate are stained 
brown by this material. Fortunately, manganese dioxide is 
removed by bisulphite, which reduces it still further, forming 
a soluble manganese salt. The brown stain can, therefore, be 
removed by immersion of the stained material in a solution of 
bisulphite. 

A very powerful cutting reducer is made from a solution 
of iodine in potassium iodide, to which potassium cyanide has 
been added to dissolve the silver iodide formed during reduc- 
tion. Iodine is not soluble in water but is soluble in a solution 
of potassium iodide, and to make up the reducer a few iodine 
crystals are dissolved in a 10% solution of potassium iodide, 
and five parts of this are added to one part of a 10% solution 
of potassium cyanide, making up to 100 parts with water for 
use. 

B. Proportional reducers are those which act on all parts 
of the negative in proportion to the amount of silver present 
there; hence they exactly undo the action of development, 
since during development the density of all parts of the nega- 
tive increases proportionally. A correctly exposed but over- 
developed negative should be reduced with a proportional re- 
ducer. Unfortunately, there are no single substances which 
form exactly proportional reducers, but by mixing perman- 
ganate, which is a slightly cutting reducer, with persulphate, 
which is a flattening reducer, a proportional reducer may be 
obtained. See formula (R-5) p. 50. 

C. In order to have a flattening reducer, we require one 
which acts very much more on the heavy deposits than on the 
light deposits of the negative, and which will consequently 
reduce the highlights without affecting the detail in the shad- 
ows. Only one such reducer is known, and this is ammonium 
persulphate. Ammonium persulphate is a powerful oxidizing 
agent and attacks the silver of the negative, transforming it 
into silver sulphate, which dissolves in the solution. It must 
be used in an acid solution and is somewhat uncertain in its 
behavior, occasionally refusing to act, and always acting more 
rapidly as the reduction progresses. 


AMMONIUM PERSULPHATE is a white crystalline ne 
stable when dry. It has been found in the Research Laboratory 
of the Eastman Kodak Company that the action of persulphate 
depends largely upon its containing a very small amount of iron 
salt as an impurity, and that its capricious behavior is due to 
variations in the amount of iron present. The persulphate supplied 
as an Kastman Tested Chemical may be relied upon to give a uni- 
form action in reduction. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 41 


INTENSIFICATION. 


Intensification is photographically the opposite of reduc- 
tion, the object being to increase contrast. This is done by the 
deposition of some other material on the silver image. A silver 
image, for instance, can be very much intensified by toning it 
with uranium (see page 35), the reddish-brown uranium 
ferrocyanide having very great printing strength and convert- 
ing a weak negative into one having a great effective contrast 
for printing purposes. Usually, however, intensification is 
performed by depositing silver or mercury upon the image, 
and most photographic intensifiers depend upon the use of 
mercury. : 

Mercury is a metal which forms two series of salts, the 
mercuric salts, which are in a higher degree of oxidation, and 
the mercurous salts. 

Many of the mercuric salts are insoluble in water, but mer- 
curic chloride is sufficiently soluble for practical use, and when 
a silver image is placed in a solution of mercuric chloride, this 
reacts with the silver and forms a mixture of mercurous chlo- 
ride and silver chloride. 

The bleached image, which appears white, can then be 
treated in various ways. If it is developed, for instance, both 
the silver chloride and the mercurous chloride will be reduced 
to the metal, and in addition to the silver, with which we 
started, we shall have added to every part of silver an equal 
part of mercury. Instead of using a developer we may blacken 
the image with ammonia, which forms a black mercury ammo- 
nium chloride and produces a high degree of intensification. 

MERCURY BICHLORIDE is a virulently poisonous salt, 
known popularly as ‘‘corrosive sublimate.”’ Its only use in photog- 
raphy is for intensification, and it is obtained in white, heavy 
crystals which are soluble with some difficulty in water. This may 


be obtained in satisfactory purity by ordering Eastman Tested 
Mercury Bichloride. 


For many purposes separate bleaching and redevelopment 
is inconvenient, and for this reason the EHastman Intensifier has 
been placed on the market, this consisting of a mercury solution 
in which the intensification proceeds continuously so that it 
can be stopped at any time. This does not give quite so great 
an intensification as the use of the two solutions, but it is far 
more convenient in operation. 

A very powerful method of intensification, used chiefly for 
negatives made by photo-engravers, is obtained by bleaching 

with mercuric chloride and blackening with silver dissolved in 


42 EASTMAN KODAK COMPANY. 


potassium cyanide. The use of the cyanide cuts the shadows 
very slightly at the same time that the highlights are intensi- 
fied, so that a great increase in the contrast of the negative is 
obtained. This is usually known as the ‘‘Monckhoven” Inten- 
sifier. 

The only other intensifier which calls for notice here is the 
chromium intensifier. The silver image is bleached with a solu- 
tion of bichromate containing a very little hydrochloric acid, 
bichromate being an oxidizer of the same type as permanganate 
or ferricyanide. The image is then redeveloped and will be 
found to be intensified to an appreciable extent. This inten- 
sifier has found increasing favor owing to the ease and cer- 
‘tainty of its operation. 

POTASSIUM BICHROMATE is made by the oxidation of 
chromium salts. It forms orange-red crystals, stable in air, and is 
easily soluble to a yellow solution. It is obtained in a pure form by 
crystallization. Potassium bichromate is used in photography both 
for bleaching negatives and for sensitizing gelatin, fish glue, etc. 
When gelatin containing bichromate is exposed to light it becomes 


insoluble in water and in this way images may be obtained in in- 
soluble gelatin. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 43 


CHAPTER VII. 


The Chemistry of Washing 


It may seem strange that a chapter dealing with washing 
should be inserted in a book on photographic chemistry, because 
washing is not usually regarded as a chemical operation. 
Nevertheless, the laws governing washing are distinctly chem- 
ical in their nature, and the importance of washing in photog- 
raphy justifies greater attention than is usually paid to the 
subject. 


As a general rule the object in washing negatives or prints 
is to remove from them the chemicals of the fixing bath which 
they contain. In the first place, it must be pointed out that it 
should not be necessary to wash out silver compounds but only 
the chemicals of the fixing bath. If an exhausted fixing bath 
is used silver compounds will be present during washing and 
must be very completely removed, so that if work has to be 
hurried and the time of washing must be cut down, it is most 
important that fixing should be complete. 


The best way of ensuring complete fixing is to use two 
fixing baths, and to transfer the negatives or prints to the sec- 
ond bath after they have been fixed in the first. Then, when 
the first bath begins to show signs of exhaustion and refuses 
to fix quickly, it should be replaced by the second, and the new, 
clean fixing bath should be used in the place of the second bath 
again. This procedure ensures that no material can be removed 
from the fixing bath until the first insoluble compound of silver 
and hyposulphite has been converted into the second soluble 
compound. It must be remembered that this first insoluble 
compound is invisible and that if a negative is transferred to 
the washing tank as soon as it is visibly clear, some of the in- 
soluble silver hypo compound will remain in the negative when 
itis dry. If the negative is transferred to a second fixing bath 
instead of the washing tank, this compound will be entirely 
removed and the task of washing will be much simplified. 


The rate of washing depends entirely upon the diffusion 
of the hypo out of the film into the water. This diffusion rate 
has nothing to do with solubility. The solubility of a substance 
fixes the proportion of the substance which can go into solution. 


44 EASTMAN KODAK COMPANY. 


There are a number of errors which are current concerning 
washing. It is commonly believed, for instance, that plates 
and paper can be washed more rapidly in warm water than in 
cold. This is a mistake. It is true that any salt will diffuse more 
rapidly in warm water than in cold, but when washing a photo- 
graphic material the diffusion has to take place in gelatin, 
and the warmer the water in which the gelatin is placed, the 
more it swells, and its swelling hinders diffusion in about the 
same proportion as the rise in temperature accelerates it, so 
that, as a matter of fact, washing goes on at about the same 
rate at all ordinary temperatures. 


It is sometimes stated that material which has been hard- 
ened in the fixing bath washes more slowly than material which 
has not been hardened. This, too, is incorrect. Gelatin is 
like a sponge; the effect of hardening it is to contract all the 
network of the sponge, but in so doing the gelatin as a whole 
is not contracted and there is no difference in the diffusion be- 
tween gelatin, which has not been hardened and which has 
been hardened, unless the gelatin has been dried after harden- 
ing. If a negative is thoroughly hardened in the fixing bath and 
then is dried down, it will not expand much when soaked again 
and consequently diffusion through it will be difficult, but be- 
fore drying the hardening does not affect diffusion and the 
materials which wash most quickly are those in which the gela- 
tin has not been swollen in its treatment, either in development 
or fixation, but has been kept in a firm, solid condition. 


The actual rate of washing may be understood by remem- 
bering that the amount of hypo remaining in the gelatin is 
continually halved in the same period of time as the washing 
proceeds. An average negative, for instance, will give up half 
its hypo in two minutes, so that at the end of two minutes half 
the hypo will be remaining in it, after four minutes one-quarter, 
after six minutes one-eighth, after eight minutes one-sixteenth, 
ten minutes one-thirty-second, and so on. It will be seen that 
in a short time the amount of hypo remaining will be infinitesi- 
mal. This, however, assumes that the negative is continually 
exposed to fresh water, which is the most important matter in 
arranging the washing of either negatives or prints. 


If a lot of prints are put in a tray and water allowed to 
splash on the top of the tray, it is very easy for the water on the 
top to run off again, and for the prints at the bottom to lie 
soaking in a pool of fairly strong hypo solution, which is much 
heavier than water and which will fall to the bottom of the 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 45 


tray. If the object is to get the quickest washing, washing 
tanks should be arranged so that the water is continuously and 
completely changed and the prints or negatives are subjected 
to a continuous current of fresh water. If water is of value, 
and it is desired to economize in its use, then by far the most 
effective way of washing is to use successive changes of small 
quantities of water, putting the prints first in one tray, leaving 
them there for from two minutes to five minutes, and then 
transferring them to an entirely fresh lot of water, repeating 
this until they are washed. 


The progress of the washing can be followed by adding a lit- 
tle permanganate solution to the wash water after the prints 
are taken out of it in order to see how much hypo is left in it, 
the presence of hypo being seen by decoloration of the perman- 
ganate. An even simpler test is to taste the prints. Six changes 
of five minutes each should be sufficient to eliminate the hypo 
effectively from any ordinary material. 


46 EASTMAN KODAK COMPANY. 


CHAPTER VIII. 


Formulae 


It is always best to use the formule for solutions recom- 
mended in the instructions issued by the maker for the use of 
photographic materials; these formule are often adjusted to 
the properties of the particular materials concerned and will 
give better and more certain results than can be obtained with 
any other formule. It is often convenient, however, to have 
available standard formule, and the following formule are 
therefore given: 


DEVELOPING FORMULAE FOR FILMS AND PLATES. 
STANDARD A. B. C. Pyro. (Formula D-1) 


Stock Solution A. Avoirdupois 
Sodium Bisulphite or Potassium Metonieuiente - 140 grains 
Pyro- - - ~ - 2 OZS8. 
Potassium Bromide: mel atl a 16 grains 
Water to make SL Ee ee 32 OZS. 

Stock Solution B. 

Water I RE ee PR 32 OZS. 
Sodium Sulphite (E. K. Co.) - = i mS tome 

Stock Solution C. 

Water - Sh ic) ss 32 ozs. 
Sodium Cartan ste (B, K. Co. ) - = = = -« 24 ons, 


Dissolve chemicals in order given. 
For Tray Development— 


Take 1 part of A, 1 part of B, 1 part of C, and 7 parts of 
water. 


For Tank Development— 


Take 1 part of A, 1 part of B, 1 part of C, and 25 parts of 
water. 


Two SoLuTION Pyro TRAY DEVELOPER. (Formula D-21) 
Stock Solution A. 


Sodium Fasulpbive or Potable Metabayinnas - 140 grains 
Pyro- - - - 2 ozs. 
Potassium Bromide. SM i er 16 grains 
Water to make — = oe i 2 32 ozs. 
Stock Solution B. 
Water ~ wm ait ie sts a 32 ozs. 
Sodium Sulphite (E. K. Co.) - - - - - 34 ozs. 
Sodium Carbonate (E. K. Co.) - - 2% ozs. 


For use, take 1 part of A, 1 part of B, aad 8 parts of water. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 47 


(Elon-Hydrochinon) 
TANK OR TRAY DEVELOPER. (Formula D-61A) 
Stock Solution. 


Hot Water Spout ani Bey Vine i Uy Gute ileey ace i |e 16 ozs. 
Elon - Se yy eS 45 grains 
Sodium Sulphite (B. K. Go. ‘i ere one OE Be 3 02S. 
Sodium Bisulphite- - - - - - - - 30 grains 
Hydrochinon -— - Se ee rm ete md sk 85 grains 
Sodium Carbonate (E. K. Co. pe ee OR) ai ead? ALG eraing 
Potassium Bromide - ssh Moe! Bae WY om 24 grains 
Cold watertomake - - - - - - - 32 ozs. 


For tray, use 1 part of stock solution to 1 part of water. 
For tank, use 1 part of stock solution to 3 parts of water. 
(Elon-Hydrochinon-Pyro) 


Amateur Finishers 
(Formula D-18) 


Hot Water (about ) meh cae bie ay a a aie 1 gallon 
lon-  - - - - = = 100 grains 
Sodium Sulphite (E. K. Co.) - = =: = = 12) ozs. 
Sodium Bisulphite - - - - - -  - 1650 grains 
Hydrochinon -— - a ee ye) nae 1 oz 
Sodium Carbonate oe K. Co. ) - - - = = 6% ozs 
Pyro- - = 8 eo eed = Droge 
Pan Bromide. wim! te Hew ee ot 60 grains 
Cold water tomake - - - - - = = 10 gallons 


Process TRAY DEVELOPER. (Formula D-9) 
(Hydrochinon-Caustic) 


Water - a ees ee ee 16 ozs 
Sodium Bieniohite Set ta eee i 34 OZ. 
A. 4 Hydrochinon Rem alia myn, aime th a 34 OZ. 
Potassium Bromide - - - - - = -« 34 OZ 
Watertomake - - - - - - = = 32 ozs 
B. fCold Water -— - a 32 OZS. 
Sodium Hydroxide (Caustic Soda) - - - = 1% ozs. 


Use equal parts of A and B and develop for three minutes 
at a temperature of 65° Fahrenheit. 


Process TANK OR TRAY DEVELOPER. (Formula D-11) 
ee aes 


Hot water (about 125° F.) - - - - - 64 ozs. 
Elon - = - eee ea as 60 grains 
Sodium Sulphite (E. K. Co. ) eh Eee ee fy ae ad iae 10 ozs. 
Hydrochinon -~ - - - - Ii ozs. 
Potassium Carbonate (or Sodium Carbonate) - - 8 ozs. 
Potassium Bromide - - - 34 OZ. 
Cold water to make - - ~ -  - 1 gallon 


Used at 65° Fahrenheit, in Stier tank or tray this developer 
will give very good contrast in five minutes. The developer 


48 EASTMAN KODAK COMPANY. 


is recommended for use with Process plates or films and with 
Process Panchromatic Plates. 

When less contrast is desired, the developer should be 
diluted with an equal volume of water. 


X-Ray DEVELOPER. (Formula D-19) 
(Elon-Hydrochinon) 


Hot water (about 125° EF.) =2F Sia Ve <a ee 16 ozs. 
Elon -— - Sey o =~ Sa 35 grains 
Sodium Sulphite (E. EK: Co.) - - - - - 8% ozs. 
Hydrochinon -~— - - - -  - = 140 grains 
Sodium Carbonate (E. K. Co. yO ae ae 
Potassium Bromide - male) (ees Gee, Bie 30 grains 
Cold watertomake - - - - - - - 32 ozs. 


Motion PicTURE DEVELOPER. (Formula D-23) 
Negative or Positive Film. (No. 16) 


Hot water (about 125° F.)  - - 5 gallons 
Bion Ol oF) Wie 5 me ae - 180 grains 
Sodium Sulphite (E. K. Co.) 3 Ibs. 5 ozs. 
Hydrochinon - - 8 ozs 
Sodium Carbonate (B, K. Co. 


Potassium Bromide . 


Tle Oe et 
Fee I OR i A ag i 
ae ae bas Fe 
| oe some Tver sea Gia sone Wiis ae: | 

— 

— 

owe 

No) 

° 

NS 

mn 


Citric Acid - - - 1 oz. 
Potassium Metabisulphite - - 2 ozs. 
Cold water to make - - - 10 gallons 
LANTERN SLIDE FORMULAE 
Blue Black Tones 
(Formula D-34) 
Stock Solution A. 
Hot water (about 125° FF.) =). = 96 a eee 16 ozs 
Elon - - me) ee awe ea 60 grains 
Sodium Sulphite (E. K. Co. ) mil | ae a Se ¥% o4. 
Hydrochinon -— - ee ee 4 oz 
Cold water to make - - - - - - - 32 ozs 
Stock Solution B. 
Water ~ ee ee 32 OZS. 
Sodium Carboni (BE, K. Co. ) 5 ee eee ie oz. 
Potassium Bromide - ~ = whee 30 grains 


For use, take equal cache of A ee B. | 
For softer results, dilute with an equal quantity of water. 


Warm Black Tones 


(Formula D-32) 
Stock Solution A. 


Hot water (about 125° F.) - - - - - - 16 ozs. 
Sodium Sulphite (E. K. Co. ) i ae ae 90 grains 
Hydrochinon --~ - - - - = = 100 grains 


Potassium Bromide - = 50 grains 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 49 


Me AGI his me a ee eee 10 grains 

Cold water to make oa rob) Ct mi ae Pk im che 32 ozs. 
Stock Solution B. 

Cold water - Se an eee 32 OZS. 

Sodium Carbonate (BE. <e Co, J) mes pe an a, 1 oz. 

Sodium Hydroxide (Caustic Soda) - - - - 60 grains 


DEVELOPING FORMULAE FOR PAPER. 


VELOX, AZO AND BROMIDE PAPER. (Formula D-67) 
Stock Solution 


Hot water rout ane ey mf ee ey eS 16 ozs. 
lon- - mie whe Cemeee Lee 45 grains 
Sodium Sulphite ©. K. Co. y wie halle eee et mt LL ona, 
Hydrochinon - - = = = = 185 grains 
Sodium Carbonate (B, K. Co. Vee Aas ei aes al 4 aero ER. 
Potassium Bromide - Sha air Satreer TY. 15 grains 
Cold water to make ot ne en ae - 2 ozs. 


For use: Dilute 1 to 1 for Velox; 1 to 2 for Azo and 1 to 
4 for Bromide Papers. 


FIXING BATHS. 


PLAIN Hypo Batu. (Formula F-11) 
Hypo a es de ee 16 ozs. 
Weterto- =< = = - = 5+ = |= = 64 ozs. 
Acip Fix1Inc Batu. (Formula F-1) 


Add the following hardener solution to 64 ozs. of plain 
hypo solution mixed as given before. 


Water ~ SY i Ba ei 5 ozs. 
Sodium Sulphite (E. K. ‘Go: ) fd Sea im ae he 1 o2. 
Acetic Acid (28% pure). spam ot eel ae way 3 OZS. 
Potassium Alum - ae ae ae 1 oz. 


Dissolve in the Res given. 


Acip HARDENER STOCK SOLUTION. (Formula F-1A) 


Water - tO a ee 56 ozs. 
Sodium Sulp hite - acy ches Sia a el 16 ozs. 
Acetic iver e (28% pure). Sopa ew 48 ozs. 
Potassium Alum - ti Ce eee 16 ozs. 
Water to make - Nie ke le ae 1 gallon 


For use, add 1 uae of stock solution to 8 parts of plain 
hypo solution as given before. 


REDUCERS. 
PERSULPHATE. (Formula R-1) 
Stock Solution. 


Water - = late | 32 OZs. 
Ammonium Persulphate ae ere eh mera iy Pc 2 ozs. 
Bulphuric Acid CO, P. - - - - - = = 34 dram 


50 EASTMAN KODAK COMPANY. 


For use, take one part of stock solution and add two parts 
of water. 

When reduction is complete, immerse in an acid fixing bath 
for a few minutes, then wash. 


PERMANGANATE. (Formula R-2) 
Stock Solution. 


Water - AML Pa ee 1 oz, 

Potassium Permanganate ee 24 grains 
B. Water - ior peti eet i Fea 1 oz. 

Sulphuric Acid ©. P. mie me eli ie pte ga Yj dram 


For use, take 1 dram A, 2 drams B, and 8 ozs. water. When 
the negative has been sufficiently reduced, immerse in a plain 
hypo solution, or in a fresh acid fixing bath for a few minutes, 
to remove yellow stain, after which wash thoroughly. 


FARMER’S. (Formula R-4) 
Add enough potassium ferricyanide solution to 25% plain 
hypo solution (not acid fixing bath), to make it lemon-yellow. 
After reducing wash thoroughly. 


Proportional Reducer 
(Formula R-5) 
Stock Solution. 


Water - - ts 32 ozs. 
A. 4; Potassium Perminnganhts = |) a Ee el ea 4 ozs. 
Sulphuric Acid (10% solution)-  - - - - 1% o2. 
B. fWater - - - = Se ee 64 ozs. 
Ammonium Persulphate - ‘- - 2 ozs. 


For use, take one part of A Me thies Hevte of B. When 
sufficient reduction is secured the negative should be cleared 
in a 1% solution of sodium bisulphite. Wash the negative 
thoroughly before drying. 


INTENSIFIERS. 
MeErcurY INTENSIFIERS. (Formula I-1) 
(Monckhoven.) 
Bleach the negative in the following solution until it is 
white, then wash eke 


Potassium Bromide - a a Nee 34 O%. 
Mercuric Chloride - - - - - -=- - = 34 OZ. 
Waterto- - - - 32 ozs. 


The negative can be binceaned wih 10% eniphite solution, 
developing solution, such as formula D-67 diluted 1 to 2, or 
10% ammonia, these giving progressively greater density in 
the order given. ‘To increase contrast greatly without in- 
creasing the density in the shadow portions blacken in: 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 51 


Sodium or Potassium Cyanide - - - - - VY oz. 
BOUEPTTALC = mem wm 34 OZ. 
eR a ee 32 ozs. 


In order to make this up, dissolve the cyanide and silver 
nitrate separately, and add the latter to the former, until a 
pemanent precipitate is just produced; allow the mixture to 
stand a short time and then filter. This is called Monckhoven’s 
Intensvfier. 


Curomium INTENSIFIER. (Formula I-4) 


Bleach in: 
Potassium Bichromate -~— - - -  - 1650 grains 
Concentrated pure Bpysroonlorin Acid - -  - 1-2 drams 
Waterto - - -  - 32 ozs. 


The more hydrochloric acid een ftte quicker the bleaching 
takes place and the less intensification is obtained. Wash 
until yellow stain is removed and then re-develop after ex- 
posure for a short time to diffused daylight. 


RE-DEVELOPMENT INTENSIFIER. 


Perhaps the simplest method of intensification for negatives 
consists of bleaching in the ferricyanide and bromide formula 
used for the sepia toning of prints and then blackening with 
sodium sulphide exactly as in print toning. 


TONING FORMULAE. 
SEPIA TONING—Hypo ALUM BATH. 
(Formula T-1a) 


Hot water about a us pS a is ea 96 ozs. 
Hypo * -  - 16 ozs. 
Dissolve Eerodehly, fen aud fie eailtming solution: 
Hot water (about 160° we bE een ee SS a 16 ozs. 
Potassium Alum - - - 4 ozs. 
While still hot, add ie coll agitip Bonin: 

Cold Water - me) tie a eet to 1 oz. 
Silver Nitrate Crystals ed) 60 grains 
Sodium Chloride (Table Salt) - - - - 60 grains 


When thoroughly dissolved, add bate to make 1 gallon. 


For use, pour into a tray standing in a water bath and heat 
to 120° F. Prints will tone in 12 to 15 minutes. 


ReE-DEVELOPING STOCK SOLUTION. (Formula T-7 ) 
No. Se) Solution. 


Potassium Ferricyanide- -— - - - = 2% ozs. 
Potassium Bromide a a ? 207: Be 


WORtCr ede Bie we mee tw we BA Ons. 


52 EASTMAN KODAK COMPANY. 


No. 2—Re-Developing Solution. 


Sodium Sulphide inot Bulphite) whe De ee 

Water - eat Cae Bamana ie 16 ozs. 
Prepare Bleaching Bath as eye kts 

Stock Solution a O. i = bl ke Bete he 16 ozs. 

Water - pe OR ee Bae TOU eM tine 16 ozs. 
Prepare Ra devalsrer as follows: 

Stock Solution ne. 2 Se ee a eS 4 ozs. 

Water - me hae I ae CU ioe CN ew) a 32 OZS. 

MANIPULATION. 


Immerse print in Bleaching Bath, letting it remain until 
only faint traces of the half-tones are left and the black of the 
shadows has disappeared. This operation will take about one 
minute. 


Rinse thoroughly in clean cold water as all chemicals must 
be removed. 


Place in Re-Developer Solution until original detail re- 
turns (for about thirty seconds). Immediately after the print 
leaves the Re-Developer, rinse thoroughly, then immerse it for 
five minutes in a hardening bath composed of 1 ounce of the 
hardener recommended for the acid fixing bath (page 50) 
and 16 ounces of water. Remove the print from this bath and 
wash for one-half hour in running water. The color and 
gradation of the finished print will not be affected by the use 
of this bath. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 53 


CHAPTER IX. 


Preparing Solutions 


A solution of any kind is obtained by dissolving a solid or 
a liquid in another liquid (or solid). The substance being 
dissolved is called the solute and the liquid in which it is dis- 
solved is called the solvent. The extent to which the solute is 
soluble in the solvent is called its solubility and when the sol- 
vent will hold no more of the solute it is said to be saturated. 


The degree of solubility of any chemical depends on the 
nature of the solvent and on the temperature, which should 
always be stated. 


If a saturated solution is cooled down to a lower tempera- 
ture, crystals usually form which settle out until the saturation 
point is reached at that particular temperature, though in the 
case of a substance like hypo, if all dust is excluded, crystals 
do not separate out on cooling and a so-called super-saturated 
solution is obtained. However, if a small crystal of hypo is 
added to the solution, crystals immediately form and continue 
to grow until the saturation point is reached. The best method 
of preparing a saturated solution, therefore, is to dissolve the 
chemical in hot water, cool to room temperature with shaking, 
allow to stand, and filter. 


When a chemical is dissolved in water the volume of the 
solution is usually greater than that of the water used, because 
the particles or molecules of the chemical occupy a certain 
space when in solution. In case two liquids are mixed, the 
final volume of the liquid is not necessarily equal to the sum 
of the volumes of the liquids mixed; it may be greater or it 
may be less. Thus fifty volumes of alcohol when added to 
fifty volumes of water at 70° F., produce ninety-seven volumes 
of the mixture and not one hundred. Moreover, equal weights 
of different chemicals do not occupy the same volume. 


In photography we are concerned only with the weight or 
volume of each chemical in a fixed volume of the solution, so 
that when mixing, the chemical should be dissolved in an 
amount of water appreciably less than that called for in the 
formula and then water added wp to the amount stated. 


54 EASTMAN KODAK COMPANY. 


WEIGHTS AND MEASURES. 
In photographic practice, solids are weighed and liquids 
are measured either by the Avoirdupois or the Metric system. 


The following Avoirdupois tables of weights and measures 
are used in photography: 


Wezght. Volume. é 
43714 grs. =1 ounce 60 minims =1 fluid drachm 
8 fluid drachms=1 fluid ounce 
16 ounces =1 pound 480 minims =1 fluid ounce 
16 ounces =] pint 
128 ounces =1 gallon 


When a formula is expressed in ounces and grains, etc., it 
may be converted into a metric formula by taking each gallon 
as 4 liters, each ounce fluid as 30 ces, multiplying the ounces of 
solid by 30 to express them as grams, (Note: Strictly speak- 
ing, one ounce equals 28.35 grams, but for convenience in 
calculation, the round figure, 30 grams, is used) and taking 15 
grains as one gram. Thus a developer formula for a 42- 
gallon tank will be converted as follows: 


Formula. 

Water 5 gallons 
Elon 1 02. 22 grains 
Sodium Sulphite 52 ozs. 
Potassium Metabisulphite loz. 273 grains 
Hydrochinon 4ozs. 88 grains 
Pyro ‘ 10 ozs. 
Sodium Carbonate 27 ozs. 359 grains 
Potassium Bromide 263 grains . 
Add enough water to make 42 gallons 

Conversion. 
Water 20 liters 
Elon 32 grams 
Sodium Sulphite 1560 grams 
Potassium Metabisulphite 48 grams 
Hydrochinon 126 grams 
Pyro 300 grams 
Sodium Carbonate 834 grams 
Potassium Bromide 17 grams 
Add enough water to make 168 liters 


To convert a metric formula into an Avoirdupois formula 
the process should be reversed, the grams being divided by 30, 


# 


7 


ke 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 55 


expressed as ounces, odd grams multiplied by 15, and ex- 
pressed as grains, liters divided by 4 and expressed as gallons, 
ace cubic centimeters divided by 30 and expressed as ounces of 
solution. 


It is often recommended to dissolve, say, 10 parts of a solid 
in 100 parts of water. In the case of liquids, parts should be 
taken as meaning units of volume and in the case of solids as 
units of weight. A “‘part’’ may, therefore, mean anything from 
a grain to a ton, or a minim to a gallon so long as the other 
quantities are reckoned in the same units of weight or volume. 


Thus: 
STOCK SOLUTION. 


For use: A three parts means A 15 ozs. 
Bone part means B 5 ozs. 





he formula contains both solids and liquids, if ounces 
(liquid) and ounces (solid) are substituted for ‘‘parts,’”’ the 
error involved falls within permissible limits. 

Example: 

Mix oe eon of solution according to the following 


formula; 


rt Beinn Sulphite 10 parts 
Pyro 1 part 
Water to 100 parts 


One gallon equals 128 ozs. Therefore, dissolve 10128 + 
100 = 12 4/5 ozs. of sulphite in water, add 114 ozs. of Pyro, and 
make up to 1 gallon. 


If a formula calls for, say, 5 drops of a solution, this is a 
very uncertain quantity because drops of liquid vary consid- 
erably in size. The average drop from the usual dropping 
bottle or burette measures about 1 minim, so that 5 drops may 
be considered as 5 minims. 


Many photographers are accustomed to making up their 
_ stock solutions of hypo, carbonate, sulphite, etc., by means of 
the hydrometer. This method has the advantage that in case 
the hypo, say, has become moist and contains an unknown 
amount of water, a definite reading on the hydrometer will give 


a solution of the same strength as if perfectly dry chemicals 


had been used. When a stock solution is made from moist 
chemi icals by weighing, the error caused by the presence of 
water may be as high as 25% or 50%. 


’ 


” 
e 


: & 
® e:*~» Ow 
a . & 


56 EASTMAN KODAK COMPANY. 


- The hydrometer method has the disadvantage that the ad- 
justment of a solution to the required strength takes consid- 
erable time, it does not convey any idea as to the percentage 
strength of the solution, and varies with the temperature. 
For instance, if a stock solution is made with hot water and 
this registers, say, 45 on the hydrometer, on cooling, the liquid 
may register 48 or 50. It is therefore absolutely necessary 
either to make all readings when the solutions have cooled to 
room temperature, or to prepare a table giving the variation 
of density of each solution with temperature. 


STOCK SOLUTIONS. 


A stock solution is a concentrated solution to which water 
is added before use. 


The limiting strength of solution which it is possible to 
make in any particular case depends on the solubility of the 
chemical, and as the solubility diminishes with temperature a 
solution should not be made stronger than a saturated solution 
at 40° F., otherwise, in cold weather, the substance would 
crystallize out. 


A stock solution of sodium sulphite should be made as 
strong as possible (20% of the desiccated salt) because at such 
a strength the solution oxidizes very slowly and will therefore 
keep, whereas in weaker solution, it combines with the oxygen 
in the air very readily and is then useless as a preservative. 


APPARATUS. 


For quantities up to two pounds a double pan balance 
should be used. For still larger quantities a platform scale may 
be used. For preparing small amounts of sample developers a 
small chemical balance weighing to one-tenth of a grain is 
necessary. 

For small quantities of solution conical glass flasks are the 
most suitable, for larger quantities enameled buckets. Earth- 
enware crocks are usually unsatisfactory because when the 
glaze cracks, the solutions penetrate within the pores and thus 
contaminate any other solutions subsequently mixed in them. 

A wooden stick or paddle covered with rubber tubing is the 
best form of stirrer, but a separate one should be used for each 
solution so as to eliminate the possibility of contamination. 

The paddle may also be used to measure out a definite vol- 
ume of solution in a tank or crock by marking it to correspond 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 57 


with definite volumes when held vertically. Such markings 
are only applicable, however, to the particular tank or crock 
for which the paddle was graduated, so that a separate paddle 
should be used for each tank or crock unless they are of the 
same shape and capacity. 


MIXING OPERATIONS. 


Chemicals should be weighed out and solutions prepared 
outside the dark-room and care should be taken when handling 
such substances as hydrochinon, resublimed pyro, potassium 
ferricyanide, etc., not to shake the finer particles into the air, 
otherwise they will enter the ventilating system and settle out 
on benches, negatives and prints, and cause no end of trouble 
in the way of spots and stains. 


Weigh out chemicals on pieces of paper and after trans- 
ferring to the mixing vessel do not shake the paper but drop it 
into the sink and allow water to flow over it, thus dissolving 
the dust. Larger quantities are most conveniently weighed out 
in buckets. 


For small quantities of solutions a glass graduate marked 
off in ounces should be used, for larger quantities use a bucket 
previously graduated, or mark off the inside of the tank or 
crock used for mixing. When measuring a liquid in a glass 
graduate place the eye on a level with the graduation mark and 
pour in the liquid until its lower surface coincides with this 
level. Owing to capillary attraction the liquid in contact with 
the walls of the graduate is drawn up the sides to that on view- 
ing sideways it appears as if the liquid has two surfaces. All 
readings should be made from the lower surface and at room 
temperature because a warm liquid contracts on cooling. 


The rapidity with which a substance dissolves in any sol- 
vent depends on its solubility and degree of fineness, the tem- 
perature of the solvent, and the rate of stirring. Since a chemi- 
cal is usually more soluble in hot water than in cold, the quick- 
est way of mixing a solution is to powder it up and dissolve in 
hot water with stirring. In the case of a few substances like 
common salt, which are only slightly more soluble in hot than 
in cold water, the use of hot water is of no advantage. 


Since most solutions are intended for use at ordinary tem- 
peratures, if hot water is used for dissolving, the solution must 
be cooled off again if it is required for immediate use, though 
usually the time taken to do this is less than the extra time 
which would be taken up in dissolving the chemical in cold 


58 EASTMAN KODAK COMPANY. 


water. When mixing, therefore, as a general rule, dissolve the 
chemical in as small an amount of hot water as possible, cool 
off, and dilute with cold water. 


After diluting with water, thoroughly shake the solution if 
in a bottle, or stir if in a tank, otherwise the water added will 
simply float on top of the heavier solution. 


When mixing a solution in a tank, never put the dry chem- 
icals into the tank, but always make sure that the chemicals 
are dissolved by mixing in separate buckets and filtering into 
the tank. 

In the case of anhydrous (dry) salts, such as desiccated 
sodium carbonate, sodium sulphite, etc., always add the chem- 
ical to the water and not vice versa, otherwise a hard cake wiil 
form which will dissolve only with difficulty. 


It is necessary to remove from the solution any suspended 
matter such as dirt, caused by the presence of dust in the chem- 
icals used, and also any residue or undissolved particles which 
might settle on the plates, film or paper during development. 
There are several methods of removing such particles as fol- 
lows: 


1. Allow the solution to stand and draw off or decant the 
clear supernatent liquid. This method is particularly useful 
when the suspended matter is so fine that it will pass through, 
a coarse filter. 


Since coarse particles settle quickly the rate of settling of a 
semi-colloidal sludge can usually be hastened by mixing the 
solution in hot water, because the heat tends to coagulate the 
suspension and causes the particles to cluster together. Thus 
if crystals of sodium sulphide which are brown, due to the 
presence of iron, are dissolved in hot water the colloidal iron 
sulphide coagulates and settles out rapidly, leaving a perfectly 
colorless solution. 


2. Filter the solution through fabric or filter paper. Filter- 
ing through paper is usually a slow process and the continual 
dropping of the solution exposes’it to the air, thus causing ; 
oxidation. It is usually sufficient to filter through very fine va 
cloth or muslin which has been washed thoroughly, otherwise 1a% 
the sizing matter in the fabric will be washed into the solngion, 
and settle out as a sludge. 


3. Asa modification of method 2, when mixing a quantity 
of solution in a tank, stretch a filter bag made of cloth over the 
tank, place the chemicals in the bag and allow hot water to 
flow into it. In this way the chemicals are dissolved and the 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 59 


solution filtered at the same time. A separate bag should be 
used for each solution so as to eliminate all risk of contamina- 
tion. 


4. A combination of methods 1 and 3 is the best and most 
desirable as follows: 


(a) For quantities of solution up to 5 gallons, filter 
through cloth into a bottle or crock fitted with a side tube and 
pinch cock. In this way the fine particles settle out but the 
drainage tube is sufficiently high so as not to disturb the 
sediment. 


(b) For motion picture work the best arrangement for 
mixing is to place the chemical room immediately above the 
developing room and to mix the solutions in large wooden 
vats or enameled tanks connected with lead piping to the 
developing and fixing tanks in the dark room underneath. The 
solutions can then be mixed in advance, allowed to settle and 
tested, so that only perfect solutions pass into the tanks located 
in the dark-room. 


. When mixing a chemical solution, if method 4 is not 
adopted, and especially if the solutions are not filtered, a scum 
usually rises to the surface consisting of fibers, dust, etc., which 
should be skimmed off with a towel. 


When a fixing bath has been used for some time and is 
allowed to stand undisturbed for a few days, any sulphuretted 
hydrogen gas which may be present in the atmosphere forms 
a metallic looking scum of silver sulphide at the surface of the , 
liquid, and on immersing the film this scum attaches itself to 
the gelatin and prevents the action of the fixing bath. Any 
such scum should be carefully removed before use with a sheet 
of blotting paper. 


How To M1x DEVELOPING SOLUTIONS. 


A developer usually contains four ingredients as follows: 
e developing agent (Elon, hydrochinon, pyro, 
ophenol, etc.). 


par 
The alkali (carbonates and hydroxides of sodium, 
hits um, lithium and ammonium). 


GC. The preservative (sulphites, bisulphites, and meta- 
bisulphites of sodium and potassium). 


D.. The restrainer (bromides and iodides of sodium, 
potassium and aramonium). 
oy 


- ye 


i," 






Phe 


60 EASTMAN KODAK COMPANY. 


If a developing agent like hydrochinon is dissolved in water, 
the solution will either not develop at all or only very slowly, 
and on standing it will gradually turn brown, because of what 
is called oxidation or chemical combination of the hydrochinon 
with the oxygen present in the air in contact with the surface 
of the liquid. This oxidation product is of the nature of a dye 
and will stain fabrics or gelatin just like a dye solution. 


On adding a solution of an alkali such as sodium carbonate, 
the hydrochinon at once becomes a developer, but at the same 
time the rate of oxidation is increased to such an extent that 
the solution very rapidly turns dark brown, and if a plate is 
developed in this solution it becomes stained and fogged. 


If we add a little sodium bisulphite to the brown colored 
solution mentioned above, the brown color or stain is bleached 
out and a colorless solution is obtained. Therefore, if the 
preservative is first added to the developer, on adding the 
accelerator the solution remains perfectly clear because the 
sulphite preserves or protects the developing agent from 
oxidation by the air. 


As a general rule, therefore, the preservative should be 
dissolved first. 


An exception to this rule should be observed with formule 
containing the following developing agents, Elon, Roylon, or 
Tozol. These substances are all readily soluble in hot water 
(about 125° F.) and do not oxidize rapidly. If the sulphite is 
dissolved before the Elon, as is the case with developers such 
as hydrochinon, a white precipitate often appears especially if 
the sulphite solution is concentrated. This precipitate consists 
of methyl para-aminophenol base which is relatively insoluble, 
but in combination with sulphuric acid (Elon) it is readily 
soluble. When sodium sulphite is added to Elon, the difficultly 
soluble base is precipitated. The reason why Elon does not 
precipitate with sodium carbonate is because the Elon base 
combines with it, forming a sodium salt which is readily soluble. 
When once the Elon is dissolved, however, it takes a fairly 
high concentration of sulphite to bring it out of solution again, 
though only a low concentration is required to prevent the Elon 
from dissolving. 


Some direction sheets recommend that a portion of the 
sulphite should be dissolved first in order to prevent the oxida- 
tion of the Elon,then dissolve the Elon, and then the remainder 
of the sulphite. Many workers add a little of the solid sulphite 
to the Elon when dissolving the latter. This procedure is quite 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 61 


satisfactory, though if the Elon is dissolved alone in water at 
a temperature not above 125° F., and the sulphite dissolved 
immediately afterwards, little or no oxidation products will be 
formed which would otherwise produce chemical fog. 

The alkali (usually carbonate) may be added in one of 
three ways: 

(a) Dissolve the carbonate separately and add to the 
cooled Elon-sulphite solution. There is danger, however, of 
the Elon precipitating out before the carbonate is added. 

(b) Add the solid carbonate to the Elon-sulphite solution, 
stirring thoroughly until dissolved. 

(c) After dissolving the Elon, dissolve the sulphite and 
carbonate together, cool and add to the Elon-sulphite mixture. 


Bromides and iodides are added to a developer to com- 
pensate for any chemical fog produced by the developer, or 
inherent in the emulsion. It is immaterial at what stage the 
bromide is added during the mixing. 


When mixing a developer the following rules should, there- 
fore, be followed: 

1. Dissolve the preservative first, except in the case of an 
Elon or Elon-hydrochinon developer, when the order should be 
Elon-sulphite-hydrochinon-carbonate-bromide. If a formula 
contains both sulphite and bisulphite, it is usual to dissolve 
these together, that is the bisulphite is dissolved in the same 
order as the sulphite. 

2. Make sure that one chemical is dissolved before adding 
the next. If the alkali is added before the crystals of the 
developing agent are dissolved, each crystal becomes oxidized 
at the surface and the resulting solution will give fog. 


3. Mix the developer at as low a temperature as possible. 
Do not use water above 125° F. 


4 In the case of desiccated chemicals like sodium car- 
bonate and sodium sulphite, add the chemical to the water and 
not vice versa. 


Two practical methods of mixing are possible, as follows: 
(a) Dissolve all the chemicals in one bottle or vessel by 
adding the solid chemicals to the water in the correct order 
(in the formula the ingredients should be named in the order 
in which they are dissolved). For example, to mix the fol- 
lowing formula. | 
Elon A5 grain 
Sodium Sulphite (E. K. Co.) 1% ozs. 
Hydrochinon 135 grains 


62 EASTMAN KODAK COMPANY. 


Sodium Carbonate (E. K. Co.) 2% ozs. 

Potassium Bromide 15 grains 

Water to 32 OZS. 
proceed as follows: 

Dissolve the Elon in 16 ozs. of hot water (about 125° F.), 
then add the sulphite, and when completely dissolved, add the 
hydrochinon. Finally add the carbonate and bromide and cold 
water to make 32 ozs. 


For large quantities the filter bag method should be used, 
the chemicals being placed in the bag and dissolved in the 
above order. 

(b) An alternative method is to dissolve the preservative 
and developing agent in one vessel and the carbonate and 
bromide in another, cool and mix. This method is the safest 
and best for quantity production. 


For example, to mix the following motion picture devel- 
oper, 


Sodium Sulphite 4 lbs. 
Hydrochinon 13 ozs. 
Sodium Carbonate 4 lbs. 
Potassium Bromide 3 OZS. 
Water to 10 gals. 


proceed as follows: 

Dissolve the sulphite in about one gallon of hot water, then 
dissolve the hydrochinon and filter into the tank. Then add 
one gallon of cold water to the tank, dissolve the sodium car- 
bonate and bromide in one gallon of hot water and filter this 
into the tank, immediately adding cold water up to ten gallons. 
The object of adding cold water to the tank before adding 
the carbonate is to cool off the solution before the carbonate 
is added. | 


MIXING CONCENTRATED DEVELOPERS. 


The extent to which a developer may be concentrated is 
determined by the solubility of the least soluble constituent, 
because a stock solution should usually withstand cooling to 
40° F. without any of the ingredients crystallizing out. 
Usually, the hydrochinon and Elon come out of solution on 
cooling, but by adding alcohol (grain, wood or denatured), up 
to a concentration of 10%, the crystallization is prevented, 
since the developing agents are very soluble in alcohol. 


The addition of the alcohol does not prevent the other in- 
gredients such as sodium sulphite from crystallizing out, in 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 63 


fact, the alcohol diminishes their solubility and therefore in- 
creases the tendency to come out of solution. 


A para-aminophenol-carbonate developer is difficult to pre- 
pare in concentrated form, though by adding a little caustic 
soda the solubility of the para-aminophenol is increased and a 
stronger solution can be thus prepared. 


When preparing concentrated developers it is important to 
observe carefully the rules of mixing, taking care to keep the 
temperature of the solution as low as possible if a colorless 
developer is to be obtained. 

The following formula is a typical example of a concen- 
trated developer and is prepared by dissolving the ingredients 
in the order given: 


Hot water (about 125° F.) 16 ozs. 
Elon 75 grains 
Sodium Sulphite (E. K. Co.) 21 ozs. 
Hydrochinon 34 OZ. 
Sodium Carbonate (E. K. Co.) 3% ozs. 
Potassium Bromide 38 grains 
Wood Alcohol AVY ozs. 
Cold water to make 32 OZS. 


TWo-SOLUTION DEVELOPERS. 


A two-solution developer is simply a one-solution developer 
split into two parts, one containing the carbonate and bromide, 
the other containing the developing agent and preservative so 
that the developer will oxidize less readily and therefore keep 
well. The reason why it is customary to keep a developer like 
pyro in two solutions, is because pyro oxidizes much more 
readily than Elon or para-aminophenol with a given amount of 
preservative. 


For purposes of mixing only one-solution developers need 
be considered because the same rules regarding mixing apply in 
both cases. 


DEVELOPER TROUBLES. 


In order to be able to explain the reason for any particular 
developer trouble it is necessary to understand thoroughly 
what takes place when the ingredients are mixed in the wrong 
order or if any ingredient is omitted from the formula, and 
also the effect of chemical impurities. It is impossible to indi- 
cate every possible trouble, but the more important ones may 
be listed as follows: 


64 EASTMAN KODAK COMPANY. 


1. The developer gives fog. Fog is the chief trouble, 
caused by faulty mixing. It may be due to any one of the 
following reasons: Violation of the rules for mixing, mixing 
the solution too hot, omission of the bromide, addition of 
too much carbonate or too little sulphite, the use of impure 
chemicals, etc. 


2. The solution is colored. As a general rule the devel- 
oper, when mixed, should be colorless, and if colored, the de- 
veloper should be suspected as being liable to give fog. In 
the case of a pyro developer mixed with bisulphite, which 
contains iron, the iron combines with the pyro to form an 
inky substance which imparts a dirty red color to the solution, 
although photographically it is harmless. 


If a pyro developer is mixed as two separate solutions, 
A and B, the B solution, which usually contains only carbonate 
and bromide, should be perfectly colorless, though if carelessly 
mixed in dirty vessels it may be colored brown by the presence 
of a little pyro A. 


3. If the solution does not develop, then either the devel- 
oping agent or the carbonate was omitted during mixing. 


How To Mix FIXING SOLUTIONS. 


Fixing baths may be divided into the following classes: 
1. Plain hypo solutions. 


2. Acid hypo solutions consisting of hypo with the addi- 
tion of sodium bisulphite, potassium metabisulphite, or sodium 
sulphite with acid. ; 


3. Acid hardening hypo solutions. 


1. No difficulty is usually experienced when mixing a 
plain hypo solution. When mixing a quantity of solution in a 
tank the filter bag method should be used and the hypo dis- 
solved in warm water because the temperature drops consid- 
erably while the hypo is dissolving. If a scum forms on the 
surface of the solution on standing this should be removed by 
drawing the edge of a towel across the surface. 


If a wooden cover is used for the tank, fungi often develop 
in a hypo solution and produce acid substances which tend to 
turn the solution milky. In such a case the tank should be 
thoroughly cleaned and the cover faced with sheet lead. 


A plain fixing bath, however, is seldom used because it 
gradually becomes alkaline from an accumulation of alkali 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 65 


carried over by prints and plates from the developer and this 
tends to soften the gelatin, while the image continues to 
develop in the fixing bath, so that if two prints stick together, 
more development takes place at the point of contact, causing 
uneven development. If the bath is acid, the acid kills or 
neutralizes the alkali in the developer carried over, thus pre- 
venting unevenness. 


2. All acid fixing baths contain either sodium bisulphite, 
potassium metabisulphite, or a mixture of sodium sulphite and 
some acid, and the following directions for mixing should be 
followed: 


(a) Do not add the bisulphite or acid sulphite solutions 
to the warm hypo solution. The solutions should be perfectly 
cold when mixed or the hypo will turn milky. 


Experience has shown that potassium metabisulphite has 
less tendency to produce sulphurization than sodium bisulphite, 
though for practical purposes the difference is almost negligible. 


(b) On keeping, an acid hypo solution gradually becomes 
milky, so that a stock solution of the sodium bisulphite, etc., 
should be kept and added to the plain hypo stock solution as 
required. For general purposes 1 oz. of a 50% sodium bi- 
sulphite solution is added to 20 ozs. of a 35% hypo solution. 
If any considerable excess over this amount is added, the 
hypo rapidly turns milky owing to the liberation of sulphur, 
especially in warm weather. 


3. Acid hardening baths are prepared by adding to hypo 
an acid hardening solution which contains the following in- 
eredients: 


a. An acid such as acetic, citric, tartaric, lactic, sulphuric, 
etc., which stops development. 


b. A hardening agent such as Boradahuth alum, potassium 
chrome alum or formaldehyde, 40%. 


c. A mae such as sodium sulphite or sodium 
bisulphite. 


The latter acts as a preservative in two ways: It prevents 
the formation of sulphur by the action of the acid on the hypo, 
while it also prevents the developer carried over into the fixing 
bath from oxidizing and turning brown. 


Prepare the acid hardening solution as a separate Bs 
solution and add this to the hypo solution as required. 


66 EASTMAN KODAK COMPANY. 


The order of mixing is important, as follows: 


(a) When mixing in one vessel, first dissolve the sulphite 
in warm water (about 125° F.), then add the acid and then the 
potassium alum. It is sometimes recommended to reverse the 
process, namely, dissolve the alum first, add the acid, and then 
the sulphite, but the alum dissolves more readily in the acid- 
sulphite solution. 


(b) The best method is to dissolve the alum and sulphite 
in separate solutions, cool, add the acid to the sulphite solution 
and then add the alum solution. 


If the order of mixing is reversed and the alum first added 
to the sulphite a white sludge of aluminum sulphite is formed 
which dissolves with difficulty when the acid is added. ‘There- 
fore, if after mixing, the hardener is milky and a sludge settles 
out, this is due to a relative insufficiency of acid, that is, the 
acid used was either not up to strength or too much alum or 
sulphite was added. 


With all other hardening baths the order of mixing is the 
same. 


FIXING BATH TROUBLES. 


1. Mulkiness of the fixing bath. 


Sometimes a fixing bath turns milky immediately on adding 
the hardener, and sometimes after being in use for some time. 
The milkiness may be of two kinds: 


A. Ifthe precipitate settles very slowly on standing, the 
milkiness is due to sulphur and may be due to the following 
causes: 


(a) Too much acid in the hardener. 


(b) Too little sulphite or the use of impure sulphite, in 
which case there is not sufficient present to protect the hypo 
from the acid. 


(c) High temperature. The hardener should only be 
added to the hypo solution when at room temperature. If the 
temperature of the acid fixing bath is over 85° F., it will not 
remain clear longer than a few days, even when mixed cor- 
rectly. The only remedy is to throw the bath away and mix 
fresh solution as required 


B. If the sulphurization disappears on standing for a few 
hours, and a gelatinous sludge of aluminum mee settles out; 
this is caused by: 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 67 


(a) Too little acid in the hardener. For example, sup- 
posing a formula calls for pure glacial acetic acid and 28% 
acid is used by mistake, then we have added less than one-third 
the required amount. 

(b) Too little hardener in the fixing bath. When fixing 
prints, a relatively large proportion of the developer is carried 
over to the fixing bath which soon neutralizes the acid and 
therefore permits of the formation of aluminum sulphite. In 
the same way a fixing bath with the correct proportion of hard- 
ener, when exhausted, still contains alum and sulphite but no 
acid, and these combine to form a sludge of aluminum sulphite. 


It is extremely important, therefore, to use only acid of 
known strength, because trouble is caused if we use either more 
or less acid than is called for in the formula. It has been found 
that the hardening properties of an alum acid fixing bath bear 
a relation to the tendency of the bath to precipitate aluminum 
sulphite. In other words, a bath containing an excess of 
-acid and which therefore may be used for a relatively long 
time before the aluminum sulphite precipitates, does not harden 
as well as a bath which precipitates when a much smaller 
quantity of developer is added. With such a bath containing 
a minimum of acid it is advisable to add a further quantity of 
acid as soon as a slight precipitate appears; a satisfactory 
quantity being about one-half the quantity originally present 
in the bath. A bath can usually be revived two or three 
times in this way before the fixing power of the hypo is ex- 
hausted. 

2 The bath does not harden satisfactorily. 


Insufficient hardening may be a result of (a) the use of 

inferior alum which does not contain the correct proportion of 
aluminum sulphate. (b) The presence of too much acid or 
sulphite, or an insufficient quantity of alum. On varying the 
proportions of acid, alum and sulphite in a fixing bath it has 
been found that the hardening increases as the quantity of 
alum increases. With increasing quantities of acetic acid with 
a given quantity of alum, the hardening increases to a maxi- 
mum beyond which the hardening decreases. A certain mini- 
mum quantity of acetic acid, however, is necessary to give the 
fixing bath a fairly long useful life before aluminum sulphite 
precipitates, but this quantity is usually greater than the 
quantity which produces maximum hardening. With use, 
therefore, the hardening properties of most fixing baths at 
first increase with the addition of developer to a maximum 
beyond which the hardening falls off rapidly. — 


68 EASTMAN KODAK COMPANY. 


In order to prolong the life of a fixing bath, therefore, the 
films or plates should be well rinsed in water before fixing so 
that the quantity of developer carried into the fixing bath is 
a minimum. 

Other acids than acetic are not generally to be recom- 
mended. Mineral acids, such as sulphuric, are too strong, 
while other organic acids such as citric, tartaric, etc., can only be 
used under certain very limited conditions, since they interfere 
with hardening. 


WATER. 


It is important to know to what extent the impurities pres- 
ent in water may be harmful and how these impurities may be 
removed. 


Excluding distilled water, rain water, and water from 
melted ice or snow, the following impurities may be present: 


1. Dissolved salts such as bicarbonates, chlorides and 
sulphates of calcium, magnesium, sodium and potassium. In 
case calcium salts are present and a developing formula is 
used containing sodium bisulphite or potassium metabisulphite, 
fine needle-shaped crystals of calcium sulphite are apt to 
separate out in the developer as a sludge on standing. ‘The 
sludge is harmless if allowed to settle though the developer is 
robbed of sulphite to the extent of the amount required to 
form the sludge. If the developer is agitated, the sludge will 
cause trouble by settling out on the emulsions of plates, films, 
etc. Other salts have usually little effect on a developer 
although chlorides and bromides exert a restraining action. 


Dissolved salts often cause trouble by erystallizing on the 
film after drying, and although not always visible as crystals 
to the eye, they detract from its transparency. 


2. Suspended matter in the form of dirt and iron rust 
which, if not filtered or allowed to settle, will cause spots. 


3. Slime, consisting of animal or vegetable colloidal mat- 
ter, and which is not removed by filtering. If such water is 
used for mixing solutions, the colloidal matter gradually 
coagulates and settles out in the solution as a sludge. 

4. Dissolved gases, such as air, sulphuretted hydrogen, 
etc. Water dissolves about 2% of air at 70° F. and when a de- 
veloping agent, like hydroquinone, is dissolved without the 
addition of sulphite, the oxygen present in the water combines 
with the developing agent forming an oxidation product which 
will cause chemical fog. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 69 


Sulphuretted hydrogen gas will also cause bad chemical 
fog, but the gas may be removed by boiling or by precipitation 
with lead acetate. 


Water may be purified as follows: 


1. By distillation—Distilled water should be used when- 
ever possible for mixing solutions. 


2. By boiling—This coagulates the colloidal matter and 
changes certain lime salts to the insoluble condition which 
then settle out, while dissolved gases such as air, sulphuretted 
hydrogen, etc., are removed. Therefore, unless the water 
contains an excessive amount of dissolved salts it is usually 
sufficient to boil the water and allow it to settle. 


3. By chemical treatment—lIf large quantities of water 
are required, chemical methods of purification must be em- 
ployed, though it is only possible to remove lime salts, slime 
and colloidal matter in this way. 


Excessive amounts of dissolved lime salts are very objec- 
tionable, because after washing if drops of water are allowed 
to remain on the plates or film, when the water evaporates, the 
dissolved salts in the water become visible as a white scum. 


The following methods of chemical purification may be 
adopted: 

a. Add alum to the water in the proportion of 15 grains to 
a gallon. This coagulates the slime which carries down any 
suspended particles, and the solution rapidly clears. This 
method does not remove dissolved salts, while the small amount 
of alum introduced into the water has no harmful effect on the 
developer. 

b. Add a solution of sodium oxalate until no further pre- 
cipitate forms. This method removes the calcium and mag- 
nesium salts and coagulates the slime, though sodium and 
potassium salts are left in solution. 

ec. Most of the commercial methods of water softening 
may be employed though such methods do not remove sodium 
and potassium salts. 


STORAGE OF CHEMICALS. 

Chemicals should be stored in well corked or well 
stoppered jars in a cool, dry place because most chemicals are 
affected by air, which contains oxygen, carbon dioxide gas and 
moisture. 

(a) Oxygen readily attacks such substances as sodium 
sulphite, especially in the presence of moisture, converting it 


70 EASTMAN KODAK COMPANY. 


into sodium sulphate, which is useless as a preservative. With 
crystallized sodium sulphite the sodium sulphate forms on the 
outside of the crystals as a powder, which may be washed 
off and the crystals dried. It is not easy to detect sodium sul- 
phate in desiccated sulphite except by chemical tests. 

Other substances which combine with oxygen and are, 
therefore, said to be oxidized, are sodium bisulphite and 
potassium metabisulphite and all developing agents such as 
pyro, hydrochinon, etc., which turn more or less brown, the 
extent of the color roughly indicating the degree of oxidation. 

(b) Carbon dioxide gas combines with substances like 
caustic soda and caustic potash, converting them into the cor- 
responding carbonated alkalis which are less reactive. If 
caustic soda is kept in a stoppered bottle the stopper usually 
becomes cemented fast by the sodium carbonate formed, so 
that it should be kept in a waxed corked bottle. Owing to the 
solvent action of the caustic alkalis on glass the inside of the 
glass bottle containing caustic or strongly carbonated solutions 
becomes frosted, though the amount of glass thus dissolved 
away will usually do no harm. 

(c) Certain chemicals have a strong attraction or affinity — 
for the moisture present in the atmosphere and gradually dis- 
solve in the water thus absorbed, forming a solution. This 
phenomenon is termed ‘‘deliquescense”’ and the chemicals are 
said to be ‘‘deliquescent.’’ Familiar examples are ammonium 
thiocyanate, potassium carbonate, caustic soda, caustic potash, 
sodium sulphide, uranium nitrate, sodium bichromate, etc., 
which should be stored in corked bottles and the neck dipped in 
melted paraffin wax. 

As mentioned earlier, it is difficult to prepare a solution of 
definite percentage strength from a chemical which has 
deliquesced, though it is usually sufficient to drain off the 
crystals, or to use a hydrometer, referring to a table giving the 
hydrometer readings in terms of percentage strength. 

(d) While some chemicals absorb moisture as above, 
others give up their water of crystallization to the atmosphere, 
and therefore lose their crystalline shape and fall to a powder 
and are then said to “‘effloresce,”’ the phenomenon being termed 
“efflorescence.”’? Some crystals do not contain any water and 
therefore cannot effloresce. 

A very dry atmosphere is suitable, therefore, for storing 
deliquescent salts but not for efflorescent salts. The only 
way to store chemicals is to isolate them from the air by suit- 
ably sealing. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 71 


Stock solutions and developers should be stored in either 
large glass bottles, earthenware crocks, wooden vats, or tanks 
of resistent material, and so arranged that the liquid may be 
drawn off at the side and near the bottom. 


Large glass bottles and crocks should be fitted with a right- 
angled glass or lead tube passing through a rubber stopper 
wired to the bottle, the tube being opened and closed by means 
of a pinch cock clamping on a short length of rubber tubing. 


In case a solution such as Pyro has to be stored for a long 
time and withdrawn at intervals, an absorption bottle contain- 
ing alkaline pyro may be fitted at the intake which absorbs 
oxygen from the air as it enters the bottle on withdrawing 
part of the solution. 


A battery of stock solution bottles may be arranged on lead 
covered shelves under which a large trough is placed, or, the 
floor may be so arranged as to form a sink so that in case of 
accidental breakage no serious damage is done. This precau- 
tion is of special importance in the case of hypo solutions, 
which might percolate into various rooms in a studio or lab- 
oratory and inoculate them with hypo dust, causing an epidemic 
of spots. 


TEMPERATURE. 


Most chemical reactions proceed more rapidly as the tem- 
perature is increased, and this is true of all the reactions 
involved in photography, so that developers and fixing baths 
will act much more rapidly when warm than when cold. 
Different reactions are stimulated to different extents by rise of 
temperature, and the effect of temperature can be measured 
numerically, the result obtained being termed the*‘temperature 
coefficient”’ of the reaction. 


As a general rule, the temperature coefficient is measured 
for a change of 10 degrees Centigrade, which are equivalent to 
18 degrees Fahrenheit, so that if a reaction which at 60 degrees 
Fahrenheit takes 4 minutes is completed at 78 degrees Fahren- 
heit in 2 minutes, we shoud say that it had a temperature 
coefficient of 2, the rate of reaction being doubled for a rise of 
18 degrees Fahrenheit. 


The temperature coefficient of development varies very 
much with the developing agent, being least with the develop- 
ers of high reduction potential, such as Elon, and most with de- 
velopers of low reduction potential, such as hydrochinon. 
There is one consequence of this which is rather important, 


72 KASTMAN KODAK COMPANY. 


namely, that the behavior of a mixed hydrochinon developer 
depends upon the temperature. At low temperature the hydro- 
chinon is very inert, while the Elon is not decreased in its 
rate of action to the same extent, and consequently the devel- 
oper behaves as if it contained an excess of Elon. At high 
temperatures the hydrochinon is increased in its activity far 
more than the Elon, and the situation is reversed. 


A similar principle applies to the fogging produced by 
developers. If development is continued for a sufficient time 
all developers will fog, but the fog reaction is a different one to 
that of development, and apparently has a different tempera- 
ture coefficient and one which is much higher than the tempera- 
ture coefficient of the development reaction itself. Conse- 
quently, a developer which will develop a material to a good 
density with low fog at a normal temperature, may produce 
very bad fog if the temperature is high. 


From the above it will be understood that the control of 
temperature in photography is of great importance and that 
so far as possible development and fixation should always be 
carried out at a normal temperature, a serious change in tem- 
perature involving much greater care and the risk of difficulty. 
If the temperature is too high, then trouble may be encountered 
with fog and with softening and frilling of the material, while 
if the temperature is too low, development will be delayed, 
there is danger of under-development, and fixing will be slow 
so that the greatest care must be taken to ensure thorough fixa- 
tion. Where the temperature cannot be controlled, as may be 
the case in the tropics, special measures must be taken, an 
account of which can be found in the booklet on ‘Tropical 
Development,” issued by the Eastman Kodak Company. 

Temperatures of solutions are measured either by the 
Centigrade or Fahrenheit thermometer. On the Centigrade 
scale water freezes at zero degrees and boils at 100 degrees, 
and on the Fahrenheit scale the corresponding readings are 
32 degrees and 212 degrees, so that 100 degrees C. are equiva- 
lent to 212 degrees minus 32 degrees or 180 degrees F., that i Is, 
1 degree C. is equivalent to 9/5 degrees F. 

To convert degrees Centigrade to Fahrenheits, multiply by. 
9/5 and add 32. To convert degrees Fahrenheit to Centigrade 
subtract 32 and divide by 9/5. 

In photography the Fahrenheit thermometer is almost 
universally employed. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 73 


CHAPTER X. 


Simple Chemical Tests 


It is often useful to be able to make simple chemical tests 
to identify photographic chemicals which are in unlabeled or 
doubtfully labeled containers. This is by no means a difficult 
matter if the photographer is provided with a few test tubes, 
some litmus paper, a little strong sulphuric acid and some lime- 
water, together with the chemicals usually kept by photog- 
raphers. Some source of heat is required, and if a spirit lamp 
or bunsen burner is not available an ordinary gas ring burner 
may be used. 


The following tests are taken from an article published in 
the “British Journal of Photography” in 1916: 


Put about one dram of the doubtful fluid in a test tube, or, 
if dealing with a solid, dissolve a few crystals in a dram of 
water. First smell the solution. If it is a metabisulphite or a 
bisulphite the smell of sulphurous acid will at once solve the 
puzzle. Ammonia, ammonium carbonate, a cyanide, or a sul- 
phide can, of course, be easily identified by smell in their orig- 
inal condition. If, however, the solution is odorless, test first 
with litmus paper to see if it is alkaline, acid or neutral, and 
- then add some strong sulphuric acid drop by drop and carefully 
note the result: 


1. The solution is alkaline and effervesces with the acid, 
remaining quite colorless. This indicates a carbonate or bi- 
carbonate, and the test can be confirmed by taking a drop of 
lime water on the end of a piece of glass tube and holding it in 
the test tube over the effervescing mixture. If the drop turns 
milky in appearance carbonic acid is being emitted, and, there- 
fore, the unknown solution is either carbonate or bicarbonate. 
To determine which, make another test by boiling some more 
solution without acid. After a minute or so of the boiling 
process, repeat the lime water test while continuing the boiling. 
If the drop then turns milky we are dealing with a bicarbonate, 
as a carbonate will give no effect. If lime water fails to reveal 
the presence of carbonic acid in the first test with the acid, then 
the solution is probably a caustic alkali. This can be confirmed 
by rubbing a drop of the solution on the hand, for the caustic 
alkalis have a curious slippery feeling that cannot well be 
mistaken. 


74 EASTMAN KODAK COMPANY. 


2. The solution is neutral, but behaves very much like a 
carbonate. It is then most probably an oxalate. Confirm by 
adding lime water to some fresh solution, when a precipitate is 
at once formed. Add hydrochloric acid, and the precipitate 
dissolves. Follow this with ammonia, and the precipitate comes 
down again. This shows the presence of an oxalate. 


3. The solution is acid and behaves like an oxalate. It is 
then plain oxalic acid. 


4. The solution is neutral. Gives off a strong odor of 
sulphurous acid when the sulphuric acid is added and remains 
quite clear. This indicates that we are dealing with a sulphite. 


5. The solution is neutral. When acid is added it gives 
off a strong odor of sulphurous acid and also turns milky and 
deposits sulphur. Here there is no doubt that the salt in ques- 
tion is hypo. 

6. Solution neutral.. Acid gives no color, but none of the 
preceding tests will identify the substance. It is probably 
acetate, and if so, boiling the acidified solution will give the 
odor of acetic acid. 


7. Solution neutral. As the acid runs in, a dark brown 
layer forms at the bottom of the tube. This denotes a bromide, 
and if the contents of the tube are thrown out into a damp 
developing dish, a strong smell of bromine will confirm that 
conclusion. If the smell of bromine is unfamiliar, add some 
acid to a dry crystal of known bromide and in a short time the 
characteristic odor will become apparent. 


8. The solution is neutral, and with the acid turns dark 
reddish-brown and thick, giving off red and violet vapors which 
condense on the tube into gray crystals. This indicates an 
iodide. 

9. The solution is acid at the start, and turns brown with 
the addition of sulphuric acid, but remains clear. The color 
deepens on boiling. This suggests tartaric acid. Confirm by 
adding fresh solution to one of caustic potash, when at a cer- 
tain stage a deposit of small crystals will be thrown down. 


If none of these effects is apparent we must try other 
tests. The acid test is dangerous if applied to a cyanide, but 
no such mistake should occur, as a cyanide can easily be iden- 
tified by smelling the plain solution. All the acid tests should 
be conducted by daylight in order that color changes may be 
noted clearly, and also in a well ventilated room if there is any 
chance of a bromide being the subject of the test. The fumes 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 75 


may have very unpleasant effects in a small, close room. A 
fairly long test tube is desirable, as some of the tests give very 
violent effervesence and are apt to produce an overflow of hot 
solution. 


The acid test given is quite enough to enable us to distin- 
quish the fact that we are dealing with a bromide, sulphite, car- 
bonate, etc., but we may also want to determine whether it is a 
sodium, potassium, or ammonium salt. The last named is 
easily detected by adding a strong solution of caustic soda, 
when, on warming, the smell of ammonia is obvious. To dis- 
tinguish a sodium from a potassium salt, dip a tuft of absorbent 
cotton in the solution and then hold it in a bunsen or alcohol 
lamp flame. Sodium will give an intense golden yellow color 
to the flame. Potassium will have little effect compared with 
the sodium, but if we look at the flame through blue glass it 
will be seen that the potassium makes the flame very much 
more brilliant and nearly white, while the sodium has hardly 
any effect. The distinction is quite obvious if we try the two 
salts together. Therefore, if one alkali is suspected, dip a 
second piece of cotton in a known solution of the other and 
try the two together, both with and without the blue glass. No 
mistake will then be possible. 


These simple tests pretty well cover the ground, and there 
‘are only a few more chemicals for which a test is necessary. 
The best test for mercuric chloride is with potassium iodide, a 
little of which gives a bright scarlet precipitate which dissolves 
on adding more iodide. 


Borax or boric acid in the solid form is easily distin- 
guished if we mix it with strong sulphuric acid to form a paste, 
then pour a little alcohol over it and light the alcohol. The 
flame will show peculiar yellow-green fringes which are unmis- 
takable. This test can be made in the corner of a developing 
dish. Borax gives an alkaline solution, boric acid a feebly 
acid one. Dip a piece of absorbent cotton in the solution and 
hold it in a flame. After a minute or so a green fringe to the 
flame will prove the presence of boric acid. The same effect 
will be produced with borax if a drop of dilute sulphuric acid 
(1 in 3 water) is put on the cotton. If test No. 6 fails, it is as 
well to dip a tuft of cotton in the acid solution and test for 
borax. Note that the green fringe does not appear until the 
cotton is dry and beginning to char. 


Alum gives a white, gelatinous precipitate with ammonia, 
not produced by any other common photographic chemical. 


76 EASTMAN KODAK COMPANY. 


Sodium phosphate gives a yellow precipitate with silver 
nitrate, the precipitate being soluble in either acids or am-— 
monia. . 


Citric acid or citrates give no precipitate with lime water 
when cold, but if heated a precipitate slowly appears. Use 
plenty of lime water for the test. 


Silver nitrate gives a precipitate with hydrochloric acid. 
The precipitate dissolves and disappears if we add ammonia, 
but reappears if we follow with sulphuric acid. 


INDEX TO CHEMICALS DESCRIBED 


Name Page 
(oS oe 30 
PATRAS OMB ras ch ess 30 
Ammonium Alum.......... 31 
Ammonia Solution.......... 23 
Ammonium Bromide..:..... 15 
Ammonium Chloride........ 15 
Ammonium Iodide.......... 15 
Ammonium Persulphate..... 40 
Ammonium Sulphocyanide... 33 
TOM sis aliens a 33 
ROUMIC POCASD 8s. sd. e ee 22 
CAUBUC DOdR......51..6...-- 22 
Chrome Alum (Potash)...... 30 
Diaminophenol Hydrochloride 20 
EBL a Sig. se, Snes avs os 31 
Ciacial Acetio......2....... 30 
MIGQIG CIOTIOO. Fcc ie ee 33 
Gold Sodium Chloride....... 33 
TAVOCOMENON. Sea SS. 19 
BREE aii a oie nce ieteteles « « 28 
BG ACBUAIG. oes ts... 33 
DMA NIPOLE oe bac... 33 
Mercuric Chloride.......... 42 
Monomethyl-para-aminophenol 

PAG a ds aaa se  « 20 
Para-aminophenol Oxalate . 19 
Potassium Bichromate...... 42 
Potassium Bromide......... 16 
Potassium Carbonate....... 25 
Potassium Chloride......... 16 
Potassium Chloroplatinite... 33 
Potassium Ferricyanide..... 34 
Potassium Ferrocyanide..... 34 
Potassium Iodide........... 16 
Potassium Metabisulphite... 26 
Potassium Permanganate.... 39 
Pyrogallol (Pyro)........... 19 


Red Prussiate of Potash..... 34 


Solubility at 40° F. 
100 ozs. (fluid) of solution contains— 
ozs. of the chemical. 


Mixes in all proportions. 
6.6 
6.9 

Mixes in all proportions. 
50 


Mixes in all proportions. 
Mixes in all proportions. 
40 

100 

4 
80 
35 
38 


INDEX TO CHEMICALS DESCRIBED—Continued. 


Name Page 
Silver Nitrate.............. 14 
Sodium Acetate............ 33 
Sodium Bisulphite.......... 26 
Sodium Bromide........... 16 
Sodium Carbonate (desiccated) 24 
Sodium Chloride........... 16 
Sodium Phosphate.......... 33 
Sodium Sulphide........... 37 
Sodium Sulphite............ 25 
Sodium Thiosulphate....... 28 


Sulphocyanide (Ammonium). 33 
Sulphocyanide (Potassium)... 34 
Thiocyanate (Ammonium)... 33 
Thiocyanate (Potassium).... 34 


Solubility at 40° F. 


100 ozs. (fluid) of solution contains— 
ozs. of the Chemical. 


110 


Specify “E. K. Co. Tested” 
when you order chemicals. 
It’s the easy way to eliminate 
chance—to make sure of the 
proper chemical results. 


Specify E. K. Co. Tested 


EASTMAN KODAK COMPANY, 
ROCHESTER, N. Y. 


All Dealers’. 





The Eastman Studio Scale 


It saves time— 
insures accuracy 


il 


Specially designed for the convenience of the 
photographic worker. 


There are no small, loose weights—just a 
sliding weight on a beam and the larger weights 
for ounces and fractions of ounces, avoirdupois. 
All bearings are of hardened steel; the beam is 
black with white markings; all other parts 
are nickeled. 


THE PRICE 
Eastman Studio Scale - és m 4) i 


EASTMAN KODAK COMPANY, 
ROCHESTER, N. Y. 


All Dealers’. 





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